Companion

Companions were the Doctor's closest friends. (AUDIO: Situation Vacant, No Place Like Home) Such people knew the Doctor's "secret": that they were someone non-human who travelled in time and space in a police box-shaped craft called the TARDIS. The Doctor's companions often directly saved the Doctor's lives (TV: Rose, The Family of Blood) or provided the Doctor with a perspective that prevented them from abusing their Time Lord powers. (TV: School Reunion, The Runaway Bride, The Fires of Pompeii) On some occasions, they were the proximate reason that the Doctor sacrificed their then-current life and regenerated. (TV: The Caves of Androzani, The Parting of the Ways, The End of Time) On others, they lost their lives in pursuit of the Doctor's goals. (TV: "The Traitors", "The Destruction of Time", Earthshock, Voyage of the Damned, Face the Raven, AUDIO: Absolution, To the Death)

Because the term was a shortening of the expression "travelling companion", most companions did in fact voyage with the Doctor in the TARDIS. Depending on the situation, other words were used to describe the same relationship — most frequently, "assistant". However, it was unusual to hear the Doctor call their friends by either word. Different incarnations had different preferences. The Third Doctor, perhaps owing to his unique situation of exile, most frequently used the term "assistant". (TV: The Ambassadors of Death, Terror of the Autons) On the other hand, when the Eighth Doctor was forced to define the word companion, he eschewed the term assistant in favour of friend. (AUDIO: Situation Vacant)

Both terms were occasionally troublesome. Sometimes, the word companion was interpreted sexually, leading to confusion about the Doctor's relationship with their friends. (TV: Aliens of London, Closing Time) Sometimes, companions bickered over the appropriateness of the word assistant. Rose Tyler said emphatically, "I'm not his assistant" when Sarah Jane called her one. (TV: School Reunion) Rose seemed happier being called a "companion", as when Harriet Jones called out over the Sub-Wave Network for "former companions of the Doctor". (TV: The Stolen Earth)

People were companions or assistants to the Doctor for varying lengths of time, but most stayed with him for more than one adventure. (TV: An Unearthly Child, The Daleks, The Rescue, The Romans, The Chase, The Time Meddler et al.) Some stayed for a few days, while others were with them for years. (AUDIO: No Place Like Home, Absent Friends) Other Time Lords, usually renegades, sometimes had travelling companions.

Humans
In the main, the Doctor travelled with humans, some from the 20th and 21st centuries. (TV: "An Unearthly Child", "Bell of Doom", The War Machines, The Wheel in Space, Rose, Dalek, School Reunion et al.) Their fondness for humans was one of the reasons they were exiled to Earth by the Time Lords. (TV: The War Games) Amongst his human companions, they tended to prefer young women — a fact the Eleventh Doctor once tried sheepishly to hide from Amy Pond. (HOMEVID: Meanwhile in the TARDIS)

Despite the presence of many female companions, they usually had platonic relationships with them. However, there were exceptions. The relationship the Ninth and Tenth Doctors had with Rose Tyler appeared romantic to other observers like Mickey Smith, (TV: Boom Town, School Reunion, Rise of the Cybermen, Doomsday) Martha Jones (TV: The Sound of Drums) and Donna Noble. (TV: Journey's End)

Their relationship with archaeologists Bernice Summerfield and River Song were more explicitly romantic. Once on Epsilon Minima, Benny unexpectedly ran into the Eighth Doctor and recalled having sex with him soon after his regeneration. (AUDIO: Benny's Story, PROSE: The Dying Days) Similarly, their relationship with River, particularly in their eleventh incarnation, was overtly romantic — to the point that he 'married' her in a ceremony that both parties seemed to accept as binding on some level. (TV: The Wedding of River Song, The Angels Take Manhattan, HOMEVID: Last Night et al.)

Romantic feelings were not always reciprocated. Martha Jones was in love with the Doctor but was rebuffed. (TV: Last of the Time Lords) Equally, Amy Pond was sexually interested in the Doctor but was firmly rejected. (TV: Flesh and Stone)

Male humans
Because female humans were statistically more common, male human companions were exceptional. The Doctor's second incarnation was never long without a male assistant and then typically only after the Time Lords forcibly removed Jamie McCrimmon from his side. (COMIC: at least Action in Exile through The Night Walkers, but possibly all TVC Second Doctor stories; PROSE: World Game, Golem, Blue Road Dance, Scientific Adviser, That Time I Nearly Destroyed the World Whilst Looking for a Dress, Mother's Little Helper, Reunion, Dust)

The Doctor's original incarnation almost always had a male companion after his introduction to Ian Chesterton. In fact, it was during his first life that the Doctor had multiple adventures exclusively with a male human, (TV: The Massacre of St Bartholomew's Eve, PROSE: Roses, Making History, Waiting for Jeremy) something that practically never happened thereafter.

Still, it did happen, as when the Fifth Doctor adventured with a World War II American pilot, Gus. (COMIC: 4-Dimensional Vistas - The Moderator) It may have happened with the Eighth Doctor's long-term companion, Fitz Kreiner. (PROSE: Escape Velocity)

According to some accounts, the Doctor also travelled solo with Jamie McCrimmon sometime after the Doctor's first Time Lord trial. However, during some of these adventures, the female companion was described as being temporarily left behind somewhere. (AUDIO: Helicon Prime) There were other accounts of the Doctor and Jamie travelling alone together, however, where no female companions were indicated. (PROSE: The Time Eater, COMIC: Invasion of the Quarks to Martha the Mechanical Housemaid) Nevertheless, it was extremely unclear how these stories "fit" into the same timestream as other established events.

Later, the Third Doctor had several one-off adventures with human males who travelled in the TARDIS with him, including Charlie Fisher, (COMIC: Backtime) Finney, (COMIC: The Planet of the Daleks) Brod, (COMIC: A Stitch in Time) and Lammers. (COMIC: The Ugrakks) Another human male companion to travel alone and for more than one adventure with this particular Doctor was a boy named Arnold. (COMIC: Children of the Evil Eye, Nova)

The Fifth Doctor had several adventures with the male Turlough after Tegan left, but before Peri arrived. However, Turlough wasn't human, but Trion. (TV: Planet of Fire) Likewise, the Sixth and Seventh Doctors each had solo adventures with the male Frobisher, but he wasn't human and most of his "solo" adventures with the Doctor took place when Peri was temporarily in New York City. (COMIC: Kane's Story)

The Eleventh Doctor briefly travelled with Rory as the only companion; Rory's wife Amy was revealed to be a Ganger during their travels after the real Amy was abducted and taken to Demons Run. After the Doctor melted Ganger Amy into raw Flesh, the Doctor and Rory concentrated their efforts on locating the real Amy and recruiting an army to fight the kidnappers. (TV: The Almost People, A Good Man Goes to War)

After spending 24 years with River Song on Darillium the Twelfth Doctor travelled with Nardole solo for a time. Their adventures included defeating Harmony Shoal. (TV: The Return of Doctor Mysterio)

Non-humans
Despite a statistical preference for humans, the Doctor had non-human companions or at the very least, companions who were not from Earth or descended from its people, like Leela. On at least three occasions they travelled with members of his own species. The longest serving were his own granddaughter (TV: An Unearthly Child, etc.) and Romana II. (TV: Destiny of the Daleks, etc.) Lady Serena was significant as well, for sacrificing her life in service to the Doctor's goals. (PROSE: World Game) Other humanoid species with whom they adventured included, but certainly weren't limited to: Trakenites, (TV: Logopolis) Alzarians, (TV: Full Circle) Cyber-converted humans (COMIC: The Company of Thieves) and people from Sto. (TV: Voyage of the Damned)

On occasion, they were accompanied by obviously non-human species, like Whifferdills, (COMIC: The Shape Shifter) Ice Warriors, (COMIC: Descendance) Oblivioners, (COMIC: Bad Blood) Eutermesans (AUDIO: The Creed of the Kromon) and Vessicans. (COMIC: Thinktwice)

Artificial life-forms sometimes travelled with the Doctor, as well. Clearly, K9 was the Doctor's most-beloved robotic companion, given the number of models the Doctor built (TV: The Invasion of Time, School Reunion, A Girl's Best Friend) and the pleasure the Tenth Doctor displayed when he re-encountered a version of the robot. (TV: School Reunion, The Wedding of Sarah Jane Smith) The Doctor seemed empathetic to artificial life-forms with errors in their programming. They once effectively euthanised an android companion in irremediable distress. (TV: Planet of Fire)

Shortly before his time on Trenzalore, the Eleventh Doctor obtained a Cyberman head he named "Handles". Handles aided him greatly with technological assistance and was his only real companion during his first three hundred years on Trenzalore as he'd sent Clara Oswald home and everyone he cared about that lived there eventually died. After three hundred years, Handles died of old age and it reduced the Doctor to tears. (TV: The Time of the Doctor)

The Twelfth Doctor teamed up with a Dalek named Lumpy to collect the Orb of Fates and stop the Daleks, Cybermen and Sontarans from controlling the Starbane. However it turned out that Lumpy had tricked the Doctor at the very beginning of the adventure. (GAME: The Doctor and the Dalek) His later companion, Nardole, was humanoid, but explicitly not human - he was at least partially cybernetic, (TV: The Pilot) harbored a distaste for humans, (TV: Smile, The Doctor Falls) and was only "human enough" because the Doctor had gotten his lungs cheap. (TV: The Pyramid at the End of the World)

Joining the Doctor
The Doctor began relationships with their companions in a great variety of ways. Some, like Adric, (TV: State of Decay) Leela, (TV: The Face of Evil) Tara, (COMIC: Official Secrets) and Zoe, (TV: The Wheel in Space) stowed away on the TARDIS, fully aware it was a ship that could take them away from their home. Others, like Ian Chesterton, Barbara Wright (TV: "An Unearthly Child") and Dodo Chaplet, (TV: "Bell of Doom") were taken away without their consent. They were also occasionally assigned assistants by those in nominal authority over him. During his exile on Earth, the Third Doctor's assistants were mostly supplied by the Brigadier. (TV: Spearhead from Space, Terror of the Autons) In the case of UNIT soldiers, the Doctor worked with whomever the Brigadier happened to have on duty at the time of a crisis. This was how he first met Mike Yates. (PROSE: The Eye of the Giant) The Doctor's other "bosses" — the Time Lords (PROSE: World Game, AUDIO: Blood of the Daleks) and the White Guardian (TV: The Ribos Operation) — also occasionally provided companions, invariably against the Doctor's wishes. Even the Daleks understood that he was commonly assisted by companions. For example, when the Daleks abducted the Eleventh Doctor for a mission regarding the Dalek Asylum, they also abducted his most recent companions, Amy Pond and Rory Williams, to assist him. (TV: Asylum of the Daleks)

All that said, the most common way of initiating TARDIS travel was to be invited by the Doctor. Although with some cases there was, from their perspective, a significant gap of time between first meeting of the Doctor and getting to travel with him, Donna Noble, (TV: The Runaway Bride, Partners in Crime) Amy Pond (TV: The Eleventh Hour) and Rory Williams (TV: The Eleventh Hour, The Vampires of Venice) were companions who began travelling at the Doctor's request a significant amount of time after they first met. In most cases there was little to no time gap between meeting the Doctor and getting to travel with them, Clara Oswald, (TV: The Bells of Saint John) Martha Jones, (TV: Smith and Jones) Rose Tyler, (TV: Rose) Jack Harkness, (TV: The Doctor Dances) Ace, (TV: Dragonfire) Vislor Turlough, (TV: Mawdryn Undead) Izzy Sinclair, (COMIC: Endgame) Charley Pollard, (AUDIO: Storm Warning) Arnold, (COMIC: Children of the Evil Eye) Jamie McCrimmon, (TV: The Highlanders) Victoria Waterfield, (TV: The Evil of the Daleks) Vicki (TV: "Desperate Measures") and others all began their travels with the Doctor because he asked them.

Other companions "forced" their way into the TARDIS. Leela, despite the Fourth Doctor's protests, ran into the TARDIS and, before he could shoo her out, fiddled with the controls, causing it to dematerialise. (TV: The Face of Evil)

Rarely, the Doctor invited people to travel with them, like Lynda Moss, (TV: The Parting of the Ways) Astrid Peth (TV: Voyage of the Damned) and Rita, (TV: The God Complex) only to have their death prevent their travels. The same thing happened with Clara Oswin Oswald, although the Doctor found out that there was another version of her somewhere in time and space and set out to find her. (TV: The Snowmen)

Leaving the Doctor
People left the Doctor for reasons as varied as the reasons they first walked through the TARDIS doors. They were of three broad types: the companion wanted to leave; (TV: "The Planet of Decision", "Horse of Destruction", The Savages, The War Machines, Last of the Time Lords) the Doctor wanted the companion to leave; (TV: "Flashpoint", The Long Game, Utopia, The God Complex) or some external force compelled the companion and Doctor to separate. (TV: "The Traitors", "The Destruction of Time", The War Games, The Hand of Fear, Earthshock, Doomsday, The Angels Take Manhattan, Hell Bent)

When asked by Brian Williams, father to one of their companions, Rory Williams, and father-in-law to Amy Pond, another companion, what happened to the people who travelled with them, the Eleventh Doctor explained that some left him, some got left behind, and also admitted that "not many but some" died. (TV: The Power of Three)

The companion's choice
The choice to leave was sometimes the companions.

A common reason for the departure of female human companions was romance. Sometimes, as with Vicki, (TV: "Horse of Destruction") Jo (TV: The Green Death) and Leela, (TV: The Invasion of Time) they left to get married, while at least one companion, Martha Jones, left because she realised her romantic feelings for the Tenth Doctor would never be reciprocated. (TV: Last of the Time Lords) In the Doctor's youth, before they had established reasonable control over the TARDIS, companions sometimes left, at least in part, because they happened to have access to their own time period. This was especially true of the companions who had not actually chosen to travel with the Doctor, such as Ian and Barbara, (TV: The Planet of Decision) Dodo, (TV: The War Machines), and Polly and Ben. (TV: The Faceless Ones) However, this sometimes happened later in the Doctor's life. Ly-Chee, for instance, thought the Seventh Doctor was only offering him a lift into town. Instead, they had several adventures together. When the TARDIS finally deposited him in the correct town, he headed for the nearest pub and ordered a stiff drink without once looking back. (COMIC: The Enlightenment of Ly-Chee the Wise)

Some companions left to improve a particular society they had encountered with the Doctor. Nyssa left to help cure Lazar's disease (TV: Terminus), whilst Steven Taylor stayed on the planet of the Elders to be its leader. (TV: The Savages) Likewise, Romana II chose to remain in E-Space to help the Tharil species. (TV: Warriors' Gate) Tara Mishra left the Ninth Doctor after wishing to stay and help with the disaster aftermath on Nomicae, (COMIC: The Bidding War) though both parties made it explicit that this was temporary, and Jack Harkness was sent to collect her after some time. (COMIC: The Lost Dimension) Later, Mickey Smith deliberately stayed in the parallel Pete's World to help its citizens defeat the Cybermen. (TV: The Age of Steel) Mickey proved especially independent when it came to departing the Doctor's side; he later left the TARDIS again, this time to return to his "home" Earth, ultimately to defend it from alien threats. (TV: Journey's End, The End of Time)

Others left for more personal improvement. Melanie Bush parted ways from the Seventh Doctor purely to have new adventures in space — but not time — with Sabalom Glitz. (TV: Dragonfire) Frobisher left the Doctor to enjoy the pleasure planet A-Lux. (COMIC: A Cold Day in Hell!) Liz Shaw left UNIT and the Third Doctor to return to her research, telling the Brigadier that all the Doctor needed in an assistant was "someone to pass him his test tubes and tell him how brilliant he was". (TV: Terror of the Autons) After another brief adventure on Messaline Martha Jones declined to travel with the Doctor again wishing to be a part of UNIT. (TV: The Doctor's Daughter)

Some companions left simply because they no longer enjoyed travelling with the Doctor. Victoria Waterfield left because she had grown tired of the continual danger in which she found herself and decided to choose a safer new life. (TV: Fury from the Deep) Tegan Jovanka similarly left because she had become sickened by the death and destruction she witnessed. (TV: Resurrection of the Daleks)

Because they were Earth-based assistants who either never wanted to travel with the Doctor on a long-term basis or never were invited to do so, a few companions made only a single journey or two, but otherwise provided significant assistance from Earth. The Brigadier (TV: The Invasion, The Five Doctors) and Wilfred Mott (TV: The Stolen Earth, The End of Time) were classic examples of this sort of assistant, but Maxwell Edison (COMIC: Stars Fell on Stockbridge) and, for a time, Mickey Smith (TV: Aliens of London/World War Three, School Reunion, The Girl in the Fireplace) could be viewed in this light as well. Harry Sullivan also left the Doctor to return to his usual life after a single trip in the TARDIS, (TV: Terror of the Zygons) albeit one punctuated by several detours by other means.

One companion who briefly left by her choice was Clara Oswald. After a trip to the Moon in which she was forced to decide whether to kill an innocent creature or not and the Twelfth Doctor refused to help, Clara grew so angry she left the TARDIS, telling the Doctor never to come find her again. (TV: Kill the Moon) However, with the help of Danny Pink, she realised she wasn't ready to give up the Doctor yet and agreed to a final trip with him weeks later to the Orient Express at which point she decided to permanently rejoin him despite the dangers and the coldness the Doctor once again displayed on the trip. (TV: Mummy on the Orient Express) Clara again left the TARDIS after Danny's death, under the mistaken belief that the Doctor had found his home planet of Gallifrey, while the Doctor believed that Danny was alive and with Clara. (TV: Death in Heaven) Later, the Doctor returned to save her life, in an adventure in which both lies were exposed. Afterwards, the Doctor gave Clara one more invitation to join him again, and she accepted. (TV: Last Christmas)

The Doctor's choice
Sometimes companions were kicked out of the TARDIS, either as punishment or for the companion's personal growth. Adam Mitchell was summarily tossed out after he was found trying to use his trip into the future to alter his own fortunes. (TV: The Long Game)

Susan, (TV: "Flashpoint") Arnold, (COMIC: The Amateur) John and Gillian (COMIC: Invasion of the Quarks) were more charitably dismissed. The Doctor clearly believed he was doing them good by ending their travels. The Doctor prevented these youths from continuing to travel with them because he felt it would interfere with their natural maturation. Married couple Amy and Rory were returned home by the Eleventh Doctor, who feared for their lives if they continued on their travels. (TV: The God Complex) The Twelfth Doctor left Clara Oswald home after believing she'd gotten her dead boyfriend Danny Pink back from the Nethersphere and would be happy with him. (TV: Death in Heaven)

Separation by situation
There were occasions when the Doctor and his companions were separated more by circumstance than the wishes of either party. Perhaps the most obvious case of this was when the Tenth Doctor and Rose Tyler were separated by the closure of the walls between dimensions. (TV: Doomsday) Sarah Jane Smith's departure occurred because the Fourth Doctor was summoned to Gallifrey (TV: The Hand of Fear) at a moment in Gallifreyan history when humans weren't allowed on the planet. (TV: School Reunion) The Time Lords forced Zoe and Jamie to part from the Second Doctor, with only the memories of their first adventure with him intact. (TV: The War Games) The Time Lords later claimed they had developed this procedure into a "standard response" for companions with whom they dealt. The Sixth Doctor, for instance, encountered a version of Peri Brown who had been given "the Jamie and Zoe treatment". She retained memories only of the adventure with the Fifth Doctor, Turlough and which began on Lanzarote. (AUDIO: Peri and the Piscon Paradox) Finally, whilst in his tenth body, the Doctor had to stabilise the effects of a biologically untenable combination of his DNA with Donna Noble's. To do so, he had to submerge all memories of her time with him and shield her from those memories lest she die. Thus, although he did, in a sense, kick her out of the TARDIS for her own good, neither wanted it. (TV: Journey's End) She did briefly remember snippets of her travels, but was knocked unconscious by it and then forgot again. (TV: The End of Time) Similarly, Donna's consciousnesses was saved to The Library against her will causing her to forget about the Doctor and spending years from her point of view married to Lee McAvoy. This was only temporary, however, and soon afterwards she resumed her travels. (TV: Silence in the Library / Forest of the Dead)

Jack Harkness' days in the TARDIS came to an end in similarly unusual circumstances. Because he had died and been resurrected by Rose as the Bad Wolf entity, he became, in the words of the Tenth Doctor "just wrong". No longer able to die, he was instead a kind of "fixed point in time" which the TARDIS herself rejected. (TV: Utopia) Nevertheless, the Doctor re-invited him on board the TARDIS (TV: Last of the Time Lords) and he later travelled aboard it without apparent difficulty. (TV: Journey's End) However, on both occasions, he was more interested in pursuing a life with Torchwood Three than returning to the Doctor's side on a long-term basis.

Amy Pond and Rory Williams were sent back in time by a Weeping Angel, separating them from the Eleventh Doctor for the remainder of their lives. They both died at age 87 and 82 respectively. (TV: The Angels Take Manhattan)

Death
Some companions died, often in service to the Doctor's goals. Katarina (TV: "The Traitors") and Sara Kingdom (TV: "The Destruction of Time") both died to prevent the Daleks obtaining taranium. Adric sacrificed himself trying to stop a prehistoric Cyberman invasion of Earth. (TV: Earthshock) Roz Forrester died in battle. (PROSE: So Vile a Sin) Lucie Miller and Tamsin Drew were both killed whilst defeating the Dalek Time Controller's invasion of Earth in the late 22nd century. (AUDIO: To the Death) Astrid Peth died killing Max Capricorn. Her atoms were left to travel the universe. Without her sacrifice, the Tenth Doctor would not have regained control of the Titanic, nor prevented it from having a costly collision into Buckingham Palace. (TV: Voyage of the Damned, Turn Left)

Cinder, who was briefly a companion of the  War Doctor , sacrificed her life to save the Doctor when the  Time Lord   Karlax  tried to  kill  him under orders from  Rassilon. It was for fear of this highly probable eventuality of a companion's death that the War Doctor was reluctant to let a companion join him during his years of fighting in the  Last Great Time War. He gave Cinder a proper burial and promised to end the Time War to honour her sacrifice. ( PROSE :  Engines of War )

Three unusual cases were those of Peri Brown, Ace and River Song.

Peri was killed by King Yrcanos on Thoros Beta, whilst her body was being subjected to Crozier's troubling body-swapping experimentations. For a time, the Sixth Doctor believed Peri had died. At the conclusion of his second major trial by the Time Lords, however, the Inquisitor revealed Peri's death had likely been a trick of the Valeyard. The Inquisitor showed the Doctor that Peri, far from being killed by Yrcanos, had, in fact, married him. (TV: Mindwarp) However, at a later date, the Time Lords revealed their meddling with Peri's timestreams had resulted in multiple versions of Peri running around the cosmos. One of these Peris had been killed by Yrcanos, as the Doctor originally believed. (AUDIO: Peri and the Piscon Paradox) Like Peri, Ace died young according to two accounts, but not others. According to one, she died while using Nitro-9 against parasitic beings known as the Lobri. Her sacrifice saved the lives of Sarah Jane, Susan, Peri and the Seventh Doctor. (COMIC: Ground Zero) In another account she was killed by George Limb, only to be replaced by a divergent version of herself, who continued to travel with the Doctor and who only had superficial differences from her other self. (PROSE: Loving the Alien) Furthermore, according to other accounts, she lived to a much older age — at least into her late thirties. (PROSE: Set Piece, amongst others)

River Song died physically when she linked her mind into the CAL computer during the Doctor's first meeting with her -- from his perspective; from hers, it was the last of many following an unspecified period of time as his companion. (TV: Forest of the Dead) Unknown to the Tenth Doctor, River was part Time Lord due to having been conceived on board the TARDIS by the Doctor's future companions Amy Pond and Rory Williams, but had given up her future regenerations to save the Eleventh Doctor's life. (TV: A Good Man Goes to War, Let's Kill Hitler) The Doctor, therefore, proceeded with his timeline knowing how River would one day die. He gave her a sonic screwdriver specially designed to save her mental patterns at the moment of her death in the Library. (TV: The Husbands of River Song) This allowed the earlier Tenth Doctor to upload River into CAL's immense memory bank, allowing her to live on in the computer. (TV: Forest of the Dead) She later managed to somehow link herself into a psychic "conference call" from inside CAL even as a data ghost to aid the Doctor one last time on Trenzalore before fading forever after he finally said goodbye to her. (TV: The Name of the Doctor)

Kamelion, an android companion, was destroyed by the Fifth Doctor in an act of mercy per Kamelion's own request after it was taken over by. (TV: Planet of Fire)

Clara Oswald, a companion of the Eleventh and Twelfth Doctors, sacrificed herself for Rigsy, whom she had only met once before. She was killed by a Quantum Shade after saying goodbye to the Time Lord, with the Doctor ultimately blaming Ashildr for her death. (TV: Face the Raven) The Twelfth Doctor extracted her from her timestream moments before her death using Time Lord technology, although she was frozen in between one heartbeat and her last. He realised that they couldn't continue travelling together as the prophecies of the Hybrid would come to pass, so he planned to erase Clara's memories of him with a neural block. However, upon discovering this, Clara attempted to reverse the polarity of the neural block so it would affect the Doctor, something he doubted had worked when she told him. They agreed to activate the device together, not knowing which one of them would be affected, and it affected the Doctor. Clara left in a stolen TARDIS with Ashildr to travel the universe before returning to Gallifrey and dying. (TV: Hell Bent)

Unknown reasons
Very rarely, it was unknown what caused certain companions to stop travelling with the Doctor. This was true of Destrii.

In the case of the Earthbound Third Doctor, relationship timings with his occasional assistants became very unclear. Assistants in this period didn't stop travelling with him; his exile on Earth made it impossible for them to start travelling with him. (TV: Spearhead from Space - The Three Doctors) Assistants during this period tended to be people who lived in England (TV: Spearhead from Space, Terror of the Autons) whom he could meet in Bessie. Since he wasn't housing companions in the TARDIS, there wasn't a definite "moment" when an assistant entered or left "the TARDIS team". Therefore, incomplete records existed as to how long the Doctor knew certain assistants. It was never known, for example, definitively when the Doctor met or said goodbye to Tom Phipps and Joe. (COMIC: The Vortex, Fogbound)

The influence of Gallifrey
The Fourth Doctor lost four consecutive companions to the direct or indirect influence of Gallifrey: he was forced to leave Sarah Jane Smith on Earth on receiving an emergency summons; (TV: The Hand of Fear) Leela and K9 Mark I chose to stay on Gallifrey; (TV: The Invasion of Time) and Romana II, who had been summoned to return to Gallifrey (TV: Full Circle) escaped by remaining in E-Space with K9 Mark II. (TV: Warriors' Gate) The Sixth Doctor was also unable to save Peri Brown, being transported away by the Time Lords at a crucial moment. (TV: Mindwarp)

Companions of other Time Lords
Other Time Lords had companions in their travels. Before his final corruption into the renegade known as the Master, the Time Lord Koschei was accompanied in his hunt for the Doctor by Ailla. Koschei believed Ailla to be a young woman from a 28th century Earth colony, but she was, in fact, a Time Lady agent sent by the High Council to spy on his increasingly erratic behaviour. (PROSE: The Dark Path) In his battle with the newly regenerated Eighth Doctor, the Master was assisted by Chang Lee, a young man in 1999 San Francisco. Lee had been convinced by the Master that the Doctor was evil. Only too late did he learn the truth when the Master killed the boy, although the TARDIS subsequently restored him to life. (TV: Doctor Who)

During the Last Great Time War, the Master took on Cole Jarnish as a companion. (AUDIO: The Good Master) However, this was nothing more than a ruse and he killed him once his usefulness had come to an end. (AUDIO: The Heavenly Paradigm), the Master worked with Chantho, who in many ways could be considered his companion. However, after regaining memories of his previous incarnations, the Master killed Chantho. (TV: Utopia)

took the Time Lord-companion relationship one step further by marrying his human companion Lucy Saxon, with every indication a passionate relationship existed between them, (TV: The Sound of Drums) only for it to turn physically abusive, culminating in Lucy shooting and apparently killing her husband. (TV: Last of the Time Lords, The End of Time)

As part of a test of the Missy incarnation of the Master's redeemed goodness, the Twelfth Doctor sent her on a mission with his own companions Bill Potts and Nardole acting as Missy's companions. (TV: World Enough and Time)

Iris Wildthyme had several companions such as Timmy, Panda, and Kroton. (PROSE: The Scarlet Empress)

When she left the Fourth Doctor in E-Space, Romana II was accompanied by K9 Mark II. (TV: Warriors' Gate) K9 continued as her companion after she returned to Normal Space. (WC: Shada, et al.) After returning to Gallifrey, Romana shared numerous adventures alongside one of the Doctor's former companions, Leela. (AUDIO: Gallifrey)

Sometime after leaving the Doctor, Lucie Miller travelled with the Monk, apparently after responding to an ad he placed in 2010 for a companion. (AUDIO: Situation Vacant) He later abandoned Lucie, instead convincing Tamsin Drew, the Doctor's current companion, to leave the Doctor and join him instead. (AUDIO: The Resurrection of Mars)

During the War in Heaven, it became commonplace for Time Lords to take their humanoid type 103 TARDISes as companions; (PROSE: Alien Bodies) equally, from another point of view, the time ships were taking their pilots as companions. (PROSE: The Book of the War) Homunculette travelled with Marie in this way. (PROSE: Alien Bodies)

In the City of the Saved, Grandfather Halfling went on extended trips to the outside universe with Amanda Legend Lefcourt (PROSE: The Book of the War) and Melicia Clutterbuck. (PROSE: Of the City of the Saved...)

Companions of non-Time Lords
After being cut off from the caldera, the 102-form timeship Compassion had multiple time sensitive companions to act as her power source. (PROSE: Warring States) These included the Gallifreyan technician Nivet (PROSE: The Ancestor Cell) and Carmen Yeh. (PROSE: The Book of the War)

When Elizabeth Klein rewrote history using the Doctor's stolen TARDIS to ensure the existence of the Third Reich into the future, she was given the position of head of Temporal Affairs. In this capacity, she travelled in the TARDIS with Major Richter, although it was an open secret that he was there to both learn how to use the TARDIS himself and ensure that Klein's loyalties remained with the Reich rather than risk her sympathizing with the aliens she encountered. (AUDIO: The Architects of History)

Jackson Lake, while believing himself to be the Doctor, had Rosita as his companion. In fact the real Doctor, in his tenth incarnation initially considered himself to be Jackson's companion. Rosita continued as Jackson's companion even after his memory and original personality were restored, being hired by Jackson to act as the nursemaid for his son Frederic Lake. (TV: The Next Doctor)

Jack Harkness compared his relationship with Angelo Colasanto to that of the Doctor and his companions. (TV: Immortal Sins) Later in his timeline, Jack's relationship with Gwen Cooper as he took her under his wing to introduce her to Torchwood also had a Doctor-companion undertone. (TV: Everything Changes)

Sarah Jane Smith developed her own cohort of companions during her time based out of 13 Bannerman Road. These were her adopted children Luke (TV: Invasion of the Bane) and Sky (TV: Sky) and several local teenagers who came and went over time: Maria Jackson, (TV: Invasion of the Bane, The Last Sontaran) Clyde Langer, (TV: Revenge of the Slitheen) Rani Chandra (TV: The Day of the Clown) and Kelsey Hooper. (TV: Invasion of the Bane) She also had two non-human companions: K9 Mark IV and the alien supercomputer Mr Smith. (TV: Invasion of the Bane)

What does the word "companion" actually mean?
The term "companion" is used more outside the programme — which is to say in fandom — than within the narrative. It was especially rarely used in the original version of the television programme and practically never uttered by the Doctor himself until the John Nathan-Turner producership. "Assistant" was a far more common designation — being used, for instance, by the First Doctor to describe Dodo Chaplet in The War Machines, by the Brigadier to introduce Jo Grant in Terror of the Autons and by the Fifth Doctor to jog the Brig's memory of Jo in Mawdryn Undead. However, "companion" has been heard with greater regularity in other media — particularly that written since the transmission of Survival.

Indeed, Big Finish Productions have an entire range of product called The Companion Chronicles, which focusses on telling Doctor Who adventures from the perspective of the assistant. Equally, Doctor Who Magazine have taken to labelling the role that of "companion" — as they did on the cover of DWM 446, when they announced the arrival of Jenna-Louise Coleman. The tacit assumption between DWM editors and their readers is that, somehow, both groups know what the other is talking about — as if the word companion were a title whose qualifications and responsibilities were well defined. However, this is not the case.

Neither assistant nor companion have ever been unambiguously defined in a narrative. Without a solid in-universe definition, viewers are left to struggle with the term on their own. At what point, they are forced to ask themselves, does a supporting character become a "companion"? Is it when they travel in the TARDIS? If so, then Liz Shaw isn't a companion, despite being the clear co-star of an entire season. Is it when they're in more than one story? If so, then Sara Kingdom, Christina de Souza, Jackson Lake, Adelaide Brooke, Wilfred Mott and Astrid Peth must be struck from the list, despite the fact that their respective actors were given star billing in their respective episodes (Kingdom excluded as she was featured in a 1960s story, long before "star billing"). Can Jackie Tyler be considered a companion, since she appeared in numerous stories and she travelled in the TARDIS, and she even helped the Doctor recover after a regeneration?

Meta-fictional and story considerations are also a problem. Canton Everett Delaware III spent three months assisting the Doctor, travelled in the TARDIS at least once, and encountered him again forty years later, yet lack of star billing and the fact his time with the Doctor is shown over only two episodes puts his status in dispute.

Then there are behind-the-scenes concerns. If an actor like Jean Marsh tells us flatly that she was not hired as a companion (BFX: The Drowned World, DOC: From Kingdom to Queen) and the official BBC website's own list of companions does not include her, is it reasonable to consider Sara Kingdom a companion? If her, why not Bret Vyon, who also appeared in several episodes of TV: The Daleks' Master Plan fulfilling a function nearly identical to Sara? In a similar vein, the early story TV: The Keys of Marinus features two characters, Altos and Sabetha, who non-ambiguously joined the TARDIS crew as companions for the course of the single adventure, although they never rode in the TARDIS itself; do they have as valid a claim to being companions as Christina de Souza, who likewise never set foot in the TARDIS during her single adventure?

This question can be expanded to the literally hundreds of characters depicted on TV, in comic strips and in literature who over the years have filled companion-like roles in one-off stories as they encountered the Doctor. Indeed some of these characters have been promoted to ersatz companion status. For a time, Henry Gordon Jago and George Litefoot could be described as just this; one-off characters from TV: The Talons of Weng-Chiang who decades later, were featured in an edition of the Big Finish Productions audio series, The Companion Chronicles, and later given their own spin-off series. However, they travelled with the Sixth Doctor in AUDIO: Voyage to Venus and Voyage to the New World, making their status as companions less controversial.

All these question marks! Determining who is a companion and who is not is one of the most common fan debates. It is possible by the fact that the television programme itself offers no definition for the term. Though probably exacerbated by the BBC Wales version's greater narrative flexibility, the debate is hardly a new one. It's been going on for ages, fuelled in previous decades by officially licensed reference works that helped mould fan opinion. For instance, the Brigadier, one of the people whose companion status is most hotly contested, gets some support for his alleged "companion status" from the book The Making of Doctor Who, in which two of the main writers of his era of the programme unambiguously called him a companion. Likewise, the later John Nathan-Turner book, The Companions was influential in making 1980s fans remember Sara Kingdom and for enshrining her as a companion — albeit against evidence in the BBC archives.

Still, though the new series put the term "companion" on more solid narrative ground — even two British Prime Ministers had used it by the end of the RTD era — it greatly confuses the definition. ". . . what constitutes a Doctor Who companion is no longer clear. Sure, Rose, Martha and Donna were all companions. So was Captain Jack. But what about Mickey and Jackie? How do you qualify? Name in the opening credits, regular trips in the Tardis? The Doctor kisses you? I'm no longer sure. Modern TV drama is so difficult."

- Steven Brook, The Guardian

In truth, though, it's never been very clear, even from a behind-the-scenes perspective. Though BBC Wales have confused things by putting single-episode guest stars in the opening credits, the "classic" series sowed similar confusion by often not listing "companions" very high in the credit list. Any definition of companion as "co-star" falls apart in the classic era, because companions were often listed after guest stars — sometimes after several guest stars. And claims that they were "regulars" must be counterbalanced against the fact that they were often contracted only for a matter of weeks, and lived under the threat of being unceremoniously dismissed mid-serial, like Jackie Lane was.

Term as used on this wiki
This narrative uncertainty makes it difficult for this wiki to consistently use the term. Several forum debates have raged as to how the term should be applied, including Disputed Companions and Who counts as a companion?. Those readers wishing to understand why certain companions are included on certain lists that appear on this site may want to explore those discussions.

Televised longevity
In televised Doctor Who, actors who played companions rarely stayed more than a series. Though some companions had notably long runs, they were the exception more than the rule. Discounting companions like Astrid Peth and Sara Kingdom — each of whom appeared in only a single story — the record-holder for shortest run would be Adam Mitchell. He appeared in only two stories or about 90 minutes of televised Doctor Who, spread across eight days of real-world time. Record-holders for longest tenures can be measured in different ways.
 * In terms of consecutive individual episodes, Jamie McCrimmon is the easy winner with one hundred and thirteen episodes between The Highlanders and The War Games, outdistancing most Doctors, much less all other companions. He served for the better part of three seasons, during which Doctor Who had its highest episode count. Due to changed television consumption habits, that record will almost certainly never be approached, much less broken. (Note that the above number doesn't take into account episodes where he's travelling with the Doctor but actors Frazer Hines or Hamish Wilson don't appear on screen, such as episode 4 of The Enemy of the World. The number increases to one hundred and sixteen if one includes his later return in The Two Doctors; Hines' appearance as Jamie in The Five Doctors is not included as he was only playing an apparition and not the actual character).
 * In terms of stories, given that BBC Wales has a much higher story count than the original series, the high water mark has been set by Clara Oswald, appearing in 30 full-length televised stories from The Snowmen on 25 December 2012 to her exit in Hell Bent on 5 December 2015. Actor Jenna Coleman also appeared as an echo of Clara prior to The Snowmen in Asylum of the Daleks.
 * In terms of calendar time that the public would have perceived a character as being a regular on Doctor Who, the winner is Tegan with a period just thirteen days shy of three years from her first regular appearance in Logopolis on 28 February 1981 to her last regular appearance in Resurrection of the Daleks on 15 February 1984, narrowly beating Clara by a mere seven days. Note that calendar years are not the same thing as seasons, as Doctor Who has not consistently begun its seasons on the same date or even month each year.
 * In terms of season count, Tegan and Sarah Jane Smith are, as of 2017, the only travelling companions to appear during four consecutive seasons as a regular, and only Jo Grant and Sarah Jane have appeared in three complete seasons. Special mention should also be given here to the character of the Brigadier, who was an incredibly long-serving member of the cast. Regular or not, the character appeared in more episodes of the programme than all of the 1980s Doctors except Tom Baker.