Companion

Companions were the Doctor's closest friends. (AUDIO: Situation Vacant, No Place Like Home) Such people knew the Doctor's "secret": that he was someone non-human who travelled in space and time in a police box-shaped craft called the TARDIS. They often directly saved his life (TV: Rose, The Family of Blood) or provided him with a perspective that prevented him from abusing his Time Lord powers. (TV: The Runaway Bride, The Fires of Pompeii, The Waters of Mars) On some occasions, they were the proximate reason that the Doctor sacrificed his 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, AUDIO: To the Death)

Because the term was a shortening of the expression "travelling companion", most companions did in fact voyage with him 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 his 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, the Eighth Doctor — whilst running a competition to find a new companion — was forced to define the word companion and 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 his 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 him for years. (AUDIO: No Place Like Home)

Other Time Lords, usually renegades, sometimes had travelling companions. These companions were not nearly so well-documented as the Doctor's.

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.) His fondness for humans was one of the reasons he was exiled to Earth by the Time Lords. (TV: The War Games) Amongst his human companions, he tended to prefer young women — a fact he once tried sheepishly to hide from Amy Pond. (HOMEVID: Meanwhile in the TARDIS)

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

His 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, his relationship with River, particularly in his eleventh incarnation, was overtly romantic — to the point that he married her in a dubiously legal ceremony that both parties nevertheless 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)

Sarah Jane Smith, who admitted to Rose and later implied to Maria Jackson that she was unable to find a man capable of taking the Doctor's place in her life, may equally have been romantically rejected by the Doctor. However, unlike the cases of Amy and Martha, there was no specific evidence that she attempted to have a romance with the Doctor, nor that he rejected her. (TV: School Reunion, Invasion of the Bane)

Male humans
Because female humans were, statistically, the most common sort of companion, 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 some time 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. (TV: The Two Doctors, 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.

All of Jamie's solo adventures differed from the situation with Steven, where there was simply a clear gap between Sara Kingdom's death and Dodo Chaplet's arrival. Later, the Third Doctor would have several one-off adventures with human males who travelled in the TARDIS with him, including Charlie Fisher, Finney, Tom, Joe, Dave Lester and Harry Trant. (COMIC: Backtime, The Planet of the Daleks, The Vortex, Fogbound, The Plant Master, The Hungry Planet) The last known 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 Demon's 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)

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 he travelled with members of his own species. The longest serving were his own granddaughter and Romana II. (TV: An Unearthly Child, The Ribos Operation, et al.) 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 he adventured included, but certainly weren't limited to: Trakenites, Alzarians, Cyber-converted humans and people from Sto. (TV: Logopolis, Full Circle, COMIC: The Company of Thieves, TV: Voyage of the Damned)

On occasion, he was accompanied by obviously non-human species, like Whifferdills, Ice Warriors, Oblivioners, Eutermesans and Vessicans. (COMIC: The Shape Shifter, Descendance, Bad Blood, AUDIO: The Creed of the Kromon, 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 he displayed when he re-encountered a version of the robot. (TV: The Wedding of Sarah Jane Smith) The Doctor seemed empathetic to artificial life-forms with errors in their programming. He once effectively euthanised an android companion in irremediable distress. (TV: Planet of Fire)

Joining the Doctor
The Doctor began relationships with his companions in a great variety of ways. Some, like Adric, Leela and Zoe, stowed away on the TARDIS, fully aware it was a ship that could take them away from their home. (TV: State of Decay, The Face of Evil, The Wheel in Space) Others, like Ian Chesterton, Barbara Wright and Dodo Chaplet, were taken away without their consent. (TV: "An Unearthly Child", "Bell of Doom") He was also occasionally assigned assistants by those in nominal authority over him. During his exile on Earth, the 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 and the White Guardian — also occasionally provided companions, invariably against the Doctor's wishes. (PROSE: World Game, AUDIO: Blood of the Daleks, TV: The Ribos Operation) Even some of the Doctor's enemies 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. Though there was, from their perspective, a significant gap of time between first becoming aware of the Doctor and getting to travel with him, Donna Noble, Amy Pond and Rory Williams were companions who began travelling at the Doctor's request. (TV: The Runaway Bride, Partners in Crime, The Eleventh Hour, The Vampires of Venice) Likewise, Martha Jones, Rose Tyler, Jack Harkness, Ace, Vislor Turlough, Izzy Sinclair, Charley Pollard, Arnold, Jamie McCrimmon, Victoria Waterfield, Vicki and others all began their travels with the Doctor because he asked them. (TV: Smith and Jones, Rose, The Doctor Dances, Dragonfire, Mawdryn Undead, COMIC: Endgame, AUDIO: Storm Warning, COMIC: Children of the Evil Eye, TV: The Highlanders, The Evil of the Daleks, "Desperate Measures")

Rarely, the Doctor invited people to travel with him, like Lynda Moss, Astrid Peth and Rita, only to have their death prevent their travels. (TV: The Parting of the Ways, Voyage of the Damned, The God Complex) The same thing happened with Clara 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)

When asked by Brian Williams, father to one of his companions and father in-law to another, what happened to the people who travelled with him, 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 companion's.

A common reason for the departure of female human companions was romance. Sometimes, as with Vicki, Jo and Leela, they left to get married, (TV: "Horse of Destruction", The Green Death, The Invasion of Time) 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 he 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, Barbara, Dodo, Polly and Ben. (TV: "The Planet of Decision", The War Machines, 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 chose to remain in E-Space to help the Tharil species. (TV: Warriors' Gate) 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)

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 and Wilfred Mott were classic examples of this sort of assistant, (TV: The Invasion, The Five Doctors, The Stolen Earth, The End of Time) but Maxwell Edison and, for a time, Mickey Smith could be viewed in this light as well. (COMIC: Stars Fell on Stockbridge, TV: Aliens of London/World War Three, The Girl in the Fireplace)

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, Arnold, John and Gillian 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 him because he felt it would interfere with their natural maturation. (TV: "Flashpoint", COMIC: The Amateur — Third Doctor version only, Invasion of the Quarks) 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) However, unlike most companions, they returned to travelling with him after having a few adventures where they returned home afterwards. Eventually both the Doctor and the two couldn't give up their adventures with each other and Amy and Rory returned to full-time companion status briefly before being separated forever in tragic circumstances beyond the Doctor's control. While Rory was first separated against his will, Amy subsequently left on purpose in hopes to join her beloved husband in a life of peace where the Doctor is unable to ever see them again. (TV: Asylum of the Daleks, Dinosaurs on a Spaceship, The Power of Three, The Angels Take Manhattan)

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)

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 onboard 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.

Death
Some companions died, often in service to the Doctor's goals. Katarina and Sara Kingdom both died to prevent the Daleks obtaining taranium. (TV: "The Traitors", "The Destruction of Time") 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. 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)

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 Threshold agents 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) Further more 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. At some point, however, he gave her a sonic screwdriver specially designed to save her mental patterns at the moment of her death in the Library. 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)

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)

One of the most unique companions the Doctor had to die was Rory Williams. Rory sacrificed himself to save the Doctor (TV: Cold Blood) but was resurrected as an Auton who acted as a companion to the Doctor briefly and was later restored as a full human being and companion after the Big Bang Two (TV: The Pandorica Opens, The Big Bang) He later created a paradox with his wife Amy Pond to destroy the Weeping Angels by sacrificing himself before they could send him back in time. He survived, but one of the Angels that made it through this incident sent him back in time to feed on his temporal energy, with the Doctor unable to retrieve him. Amy allowed the Angel to make contact with her to reach Rory. Both then died of old age (82 and 87 respectivly) at some point between when they got transported back and the present time as a grave near where they were transported from reflected, having never again been able to meet the Doctor as they had created a fixed point in time. (TV: The Angels Take Manhattan)

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. Assistants during this period tended to be people who lived in England 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.

The Gallifrey influence
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 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 Doctor subsequently restored him to life. (TV: Doctor Who)

As Professor Yana, 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)

The Master 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 Time Lady known to the Doctor as Iris Wildthyme had several companions such as Timmy, Frobisher and Kroton. (PROSE: The Scarlet Empress)

When she left the Fourth Doctor in E-Space, Romana 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)

Some time 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 her, convincing Tamsin Drew, the Doctor's current companion, to join him instead. (AUDIO: The Resurrection of Mars)

Companions of non-Time Lords
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. (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 and Sky and several local teenagers who came and went over time: Maria Jackson, Clyde Langer, Rani Chandra and Kelsey Hooper. She also had two non-human companions: K9 Mark IV and the alien supercomputer Mr Smith. (TV: Invasion of the Bane, Revenge of the Slitheen, The Last Sontaran, The Day of the Clown, The Nightmare Man, Sky)

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.

That's not the case.

Neither assistant nor companion have ever been unambiguously defined in a narrative. Since it's explicitly about the Eighth Doctor attempting to find a new companion, Situation Vacant undoubtedly comes the closest, but even its efforts are fairly permeable. 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 lots of 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 Delaware 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 (AUDIO extras: 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, most notably Henry Gordon Jago and George Litefoot, 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.

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. There, 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 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 — record-holders for shortest run include Katarina and Adam Mitchell. Each appeared in only about two hours of televised Doctor Who. Dodo Chaplet also had an atypically short run of only sixteen twenty-five-minute episodes. Some companions who seemed to be around for a fair number of seasons, in fact had relatively few stories to their name. The classic example is Ace, who was only in nine stories — one fewer than the number of stories in the typical BBC Wales series - despite appearing in two complete seasons. Polly and Ben were in as many adventures in less than half the time. Because Ace was only in a mere thirty-one episodes, even Susan easily bests her. Only two companions, Jo Grant and Sarah Jane Smith have appeared in three complete seasons. 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 TV: 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 Frazer Hines or Hamish Wilson doesn't appear on screen. The number increases to one hundred and sixteen if one includes his later return in TV: The Two Doctors; Hines' appearance as Jamie in TV: 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 so far been set by Rose Tyler, having been the primary companion for 31 episodes between series 1 and series 4, as well as Children Need 2005 plus various Tardisodes. Also she has had cameos in various episodes including Partners in Crime, The Poison Sky, Midnight and The End of Time - Part Two. This episode total has been matched by Amy Pond during her tenure from the start of series 5 to her departure in series 7. In addition, she has appearances in televised mini-episodes, DVD bonus features, web specials, The Adventure Games, and her cameos in Closing Time and The Doctor, the Widow and the Wardrobe.
 * In terms of calendar time that the public would have perceived a character as being a regular on Doctor Who, the winner is Tegan. She clocks in at three days shy of three years, from her first to last regular appearances. She only very narrowly defeats Sarah Jane, who is some six weeks shy of three years as a regular character. Note that calendar years are not the same thing as seasons, because Doctor Who did not begin its seasons on the same date each year (it should be noted, however, that Tegan and Sarah Jane are as of 2012 the only travelling companions to appear during four consecutive seasons as a regular). Special mention should be given here, too, to the character of the Brigadier. It's entirely possible that some members of the viewing community regarded him as at least a kind of regular throughout all five seasons of Jon Pertwee's Doctor Who. Although this isn't true from a contractual standpoint, he's still 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.
 * In terms of how long a companion has remained with the Doctor within the story, Amy Pond and Rory Williams seem to hold the record, having travelled with the Doctor on and off for roughly 10 years of their lives. Amy and Rory started their adventures with the Doctor at about 21 in The Eleventh Hour, and were in their early thirties by the time of The Angels Take Manhattan.
 * If the Doctor's TARDIS is considered as a companion, she tops all categories, having been a constant in the series since 23rd November 1963 and being ubiquitous across all media with the notable exception of the early Third Doctor era when the TARDIS was not featured regularly due to the Doctor's exile to Earth, as well as the Fourth Doctor stories The Sontaran Experiment and Genesis of the Daleks and the Tenth Doctor story Midnight.