Companion


 * ''You may be looking for a list of companions or a list of their appearances.

Companions were the Doctor's closest friends. (BFA: Situation Vacant, DW: Day of the Moon, Let's Kill Hitler) Such people knew the Doctor's "secret" — namely 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 (DW: Rose, Dalek, Smith and Jones, The Family of Blood) or provided him with a perspective that prevented him from abusing his Time Lord powers. (DW: 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. (DW: 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. (DW: The Daleks' Master Plan, Earthshock, Voyage of the Damned, BFA: 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 could be used to describe the same relationship — most frequently, "assistant". However, it was somewhat unusual to hear the Doctor describe his friends using either term, and different incarnations seemed to have different preferences. The Third Doctor, perhaps owing to his unique situation of exile, most frequently used the term "assistant". (DW: 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. (BFA: Situation Vacant)

Both terms were sometimes problematic for others, as well. For instance, the London police once interpreted the word "companion" sexually. (DW: Aliens of London) On another occasion, two of the Doctor's companions briefly bickered over the appropriateness of the term "assistant". Rose Tyler once flatly said, "I'm not his assistant" when Sarah Jane called her one, which caused Sarah to ironically exclaim "Get you, tiger!", intimating that Rose was just fooling herself. (DW: School Reunion) However, Rose seemed happier being called a "companion", as when Harriet Jones called out over the Sub-Wave Network for "former companions of the Doctor". (DW: The Stolen Earth) People were companions or assistants to the Doctor for varying lengths of time, but most stayed with the Doctor for more than one adventure. (An Unearthly Child, The Daleks, The Rescue, The Romans, The Chase, The Time Meddler, et al.)

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

Humans
In the main, the Doctor travelled with humans, a number of whom were from the 20th and 21st centuries. (DW: An Unearthly Child, The Massacre of St Bartholomew's Eve, 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. (DW: The War Games) Amongst his human companions, he tended to prefer females — a fact he once sheepishly tried to hide from Amy Pond. (DW: Meanwhile in the TARDIS)

Male humans
Because female humans were, statistically, the most common sort of companion, male human companions were exceptional, and worthy of more detailed study. 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. (TVC: at least Action in Exile through The Night Walkers, but possibly all TVC Second Doctor stories; PDA: World Game, ST: '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 following his introduction to Ian Chesterton. In fact, it was during his first life that the Doctor had multiple adventures exclusively with a male human, (DW: "War of God", BE: Roses, ST: Making History, Waiting for Jeremy) something that practically never happened thereafter.

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

Of course, according to some accounts, the Doctor also travelled solo with Jamie McCrimmon, some time after the Doctor's initial Time Lord trial. However, during some of these adventures, the female companion was described as being temporarily left behind somewhere. (DW: The Two Doctors, CC: Helicon Prime) There were other accounts of the Doctor and Jamie travelling alone together, however, where no female companions were indicated. (ST: The Time Eater, TVC: Invasion of the Quarks to Martha the Mechanical Housemaid) However, 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. (TVA: 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 the Doctor was a boy named Arnold. (TVC: 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. (DW: Planet of Fire) Likewise, the Sixth and Seventh Doctors both had solo adventures with the male Frobisher, but he wasn't human, and most of his "solo" adventures with the Doctor actually took place when Peri was temporarily in New York City. (DWM: Kane's Story)

The Eleventh Doctor briefly travelled with Rory as the only companion, as Rory's wife Amy was revealed to be a Ganger during their travels and 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 in locating the real Amy and recruiting an army to fight against the kidnappers. (DW: The Almost People, A Good Man Goes to War)

Near- and non-humans
Despite a statistical preference for humans, the Doctor did have non-human companions, or at the very least companions who were not from Earth. On at least three occasions he traveled with members of his own species, the most long-serving of whom were his own granddaughter and Romana — but Lady Serena was significant, as well, for sacrificing her life in service to the Doctor's goals. (PDA: World Game) Other near-human species with whom he adventured included, but certainly weren't limited to: Trakenites, Alzarians, Cyber-converted humans, and people from Sto. (DW: Logopolis, Full Circle; DWM: The Company of Thieves; DW: Voyage of the Damned'')

The Doctor did not always travel with humans, however. On occasion, he took on board the TARDIS obviously non-human species, like Whifferdills, Ice Warriors, Oblivioners, Eutermesans, and Vessicans. (DWM: The Shape Shifter; RT: Descendance; DWM: Bad Blood; BFA: The Creed of the Kromon; DWM: 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 new models the Doctor built, (DW: The Invasion of Time, School Reunion, KAC: A Girl's Best Friend) and the enthusiasm he displayed when he re-encountered a version of the robot. (SJA: The Wedding of Sarah Jane Smith) The Doctor seemed empathetic to artificial life-forms with errors in their programming, and once effectively euthanize an android companion in irremediable distress. (DW: Planet of Fire)

Joining the Doctor
The Doctor began relationships with his assistants in a wide variety of ways. Some, like Adric, Leela and Zoe actively stowed away on the TARDIS, fully aware that it was a ship that could take them away from their home environment. (DW: State of Decay, The Face of Evil, The Wheel in Space) Others like Ian Chesterton, Barbara Wright, and Dodo Chaplet were dematerialised away from their homes without their consent. (DW: "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 simply supplied by the Brigadier. (DW: 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. (MA: 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. (PDA: World Game, BFA: Blood of the Daleks; DW: The Ribos Operation)

All that said, the most common way of initiating TARDIS travel was simply 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 eventually getting to travel with him, Amy Pond and Rory Williams were companions who began travelling at the Doctor's request. (DW: The Eleventh Hour, Vampires in Venice) Likewise, Martha Jones, Donna Noble, Rose Tyler, Jack Harkness, Wilfred Mott, Ace, Turlough, Izzy Sinclair, Charley Pollard, Arnold, Jamie McCrimmon, Victoria Waterfield, Vicki and others all began their travels with the Doctor because he invited them.

Rarely, the Doctor invited people to travel with him, like Astrid Peth, only to have their death prevent their travels. (DW: Voyage of the Damned)

Leaving the Doctor
The reasons people left the Doctor's side were as varied as the reasons they first walked through the TARDIS doors. However, reasons for discontinuing a relationship with the Doctor were broadly of three types: either the companion wanted to leave, the Doctor wanted the companion to leave, or some external force compelled the companion and Doctor to be separated.

The companion's choice
The choice to leave was sometimes that of the companion.

A common reason for the departure of female human companions was romance. Sometimes, as with Vicki, Jo and Leela, they left to get married, (DW: "Horse of Destruction", "The Green Death, The Invasion of Time) while at least one companion, Martha Jones, left because she realised her romantic feelings towards the Doctor would never be reciprocated. (DW: 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 again. 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. (DW: "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 ended up on 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. (IHP: The Enlightenment of Ly-Chee the Wise)

Some companions left to improve a particular society they had encountered with the Doctor. Nyssa of Traken left to help cure Lazar's disease (DW: Terminus, whilst Steven Taylor stayed behind on the planet of the Elders in order to be its leader. (DW: The Savages) Likewise, Romana decided to remain in E-Space to help the Tharil species. (DW: Warriors' Gate) Later, Mickey Smith would deliberately choose to stay in the parallel Pete's World to help its citizens defeat the Cybermen. (DW: The Age of Steel) Mickey would prove especially independent when it came to departing the Doctor's side, as he later left the TARDIS again, this time choosing to return to his "home" Earth, ultimately to defend it from alien threats. (DW: 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. (DW: Dragonfire) Frobisher left the Doctor to enjoy the pleasure planet, A-Lux. (DWM: A Cold Day in Hell) Liz Shaw left UNIT and the Third Doctor in order to return to her research, telling the Brigadier that all the Doctor needed in an assistant was little more than "someone to pass him his test tubes and tell him how brilliant he was". (DW: 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 (DW: 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 regard as well. (DWM: Stars Fell on Stockbridge, DW: 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 ejected, after it was discovered he was trying to use his trip into the future to alter his own personal fortunes. (DW: The Long Game)

Susan, Arnold, John and Gillian were more charitably dismissed, the Doctor clearly believing 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. (DW: "Flashpoint"; TVC: "The Amateur" — Third Doctor version only, Invasion of the Quarks)

Separation by situation
There were occasions on which the Doctor and his companions became separated more by circumstance than by 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. (DW: Doomsday) There were other, similar cases. Sarah Jane Smith's departure came about because the Fourth Doctor was summoned to Gallifrey (DW: The Hand of Fear) at a moment in Gallifreyan history when humans weren't allowed on the planet. (DW: School Reunion) The Time Lords forced Zoe and Jamie to part from the Doctor, with only the memories of their first adventure with the Doctor intact. (DW: The War Games) The Time Lords would later claim that 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 the Master which began on Lanzarote. (CC: Peri and the Piscon Paradox) Finally, whilst in his tenth body, the Doctor had to stablise the effects of a biologically untenable combination of his DNA with Donna Noble's. In order to do this, he had to submerge all memories of her time with him, and to thereafter shield her from those memories being triggered — or she would die. Thus, although he did, in a sense, kick her out of the TARDIS for her own good, neither wanted to part company. (DW: Journey's End)

Death
Some companions died, often in service to the Doctor's goals. Katarina and Sara Kingdom both died while on the quest to prevent the Daleks from obtaining taranium. (DW: "The Traitors", "The Destruction of Time") Adric sacrificed himself trying to stop a pre-historic Cyberman invasion of Earth. (DW: Earthshock) Roz Forrester died in battle. (NA: 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. (BFA: To the Death) Astrid Peth died while killing Max Capricorn. Without her sacrifice, the Tenth Doctor wouldn't have been able to regain control of the Titanic nor prevent it from having a costly collision into Buckingham Palace. (DW: Voyage of the Damned)

Three unusual cases were those of Peri Brown, Ace and Adelaide Brooke.

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 that Peri had died. At the conclusion of his second major trial by the Time Lords, however, the Inquisitor revealed that Peri's death had likely been a trick of the Valeyard. Instead, the Inquisitor showed the Doctor that Peri, far from being killed by Yrcanos, had in fact married him. (DW: Mindwarp) However, at a later date, the Time Lords revealed that their meddling with Peri's timestreams had resulted in multiple versions of Peri running around the cosmos. One of these Peris had in fact been killed by Yrcanos, as the Doctor originally believed. (CC: Peri and the Piscon Paradox) Like Peri, Ace died at a young age according to one account, but not others. According to one view, she died while using Nitro-9 against Threshold agents known as the Lobri. Her sacrifice protected the lives of Sarah Jane, Susan, Peri and the Seventh Doctor (DWM: Ground Zero) According to other accounts, she lived to a much older age —at least into her late 30s. (NA: Set Piece, amongst others) Unlike Peri, however, there was no evidence of Time Lord interference in her timestream, not any indication that there were actually several different, simultaneously extant Aces.

Adelaide Brooke's death was complicated by Time Lord interference, though this time by the Tenth Doctor himself. Temporarily believing himself unshackled from the Laws of Time, he kept her alive, despite the fact that her death was a fixed point in time. She killed herself in order to correct whatever errors to the Web of Time that the Tenth Doctor had introduced by saving her life contrary to the Laws of Time. (DW: The Waters of Mars) The act shook the Doctor, causing him to at first veer into a course of debauchery, then later to investigate his own belief system and return to his traditional morals. (DW: The End of Time)

Unknown reasons
Very rarely, it was unknown what caused certain companions to stop travelling with the Doctor, as 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 during this period, 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.

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 the increasingly erratic Koschei's actions (MA: 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 as the Master killed the boy, although the Doctor subsequently restored him to life. (DW: Doctor Who)

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

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

When she left the Fourth Doctor in E-Space, Romana was accompanied by K-9 Mark II, and K-9 continuedas 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 (BFA: Gallifrey).

After saving him from the Cybermen, the Rani took Cyrian as her companion. (DWA: Rescue)

Some time after leaving the Doctor, Lucie Miller travelled with the the Monk, apparently after responding to an ad he placed in 2010 for a companion. (BFA: Situation Vacant) He would later abandon her, convincing Tamsin Drew, the Doctor's current companion, to join him instead. (BFA: The Resurrection of Mars)

Companions of non-Time Lords
Jackson Lake, while believing himself to be the Doctor, had Rosita as his companion, and in fact the real Doctor initially considered himself to be Jackson's companion. (DW: The Next Doctor)

Jack Harkness compared his relationship with Angelo Colasanto with that of the Doctor and his companions. (TW: Immortal Sins)

What does the word "companion" actually mean?
By definition, the word companion means: someone who travels willingly with The Doctor via His TARDIS. 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. "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 the BBC Wales version, beginning with the Ninth Doctor's claim to London police that he had "employed [Rose] as my companion". (DW: Aliens of London) The term is also encountered in other media, such as the Tenth Doctor's comment "Wow, I really need a companion". (IDW: Ground Control)

This conflicting approach to naming was meta-textually referenced in School Reunion, when Sarah Jane, an old-series character, called Rose the Doctor's "new assistant". Rose flatly eschewed the term, but Sarah clearly believed Rose was just fooling herself. Nevertheless, neither term is particularly ubiquitous within the 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 Christina de Souza, Jackson Lake, Adelaide Brooke, Wilfred Mott and Astrid Peth must be stricken from the list, despite the fact that their respective actors were given star billing in their respective episodes. Can Jackie Tyler be considered a companion, since she's appeared in lots of stories and she's travelled in the TARDIS, and she's even helped the Doctor recover after a regeneration? Meta-fictional and story considerations are also a problem. Canton Delaware spends three months assisting the Doctor, travels in the TARDIS at least once, and encounters him again 40 years later, but lack of star billing and the fact his time with the Doctor is shown over only two episodes puts his status in dispute. And then there are behind-the-scenes concerns. If an actor, like Jean Marsh tells us flatly that she was not hired as a companion (CC 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?

Because of all these question marks, determining who is a companion, and who is not, is one of the most common fan debates, made possible by the fact that, again, the television programme itself actually 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, fueled 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, Doctor Who: 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 confused 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

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 — both 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 25-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. Even when compared with classic series-only companions, Ace isn't all that "wicked". Polly and Ben were in as many adventures in less than half the time.Because Ace was only in a mere mere episodes, even Susan easily bests her. Record-holders for longest tenures can be measured in a number of different ways.
 * In terms of consecutive individual episodes, Jamie McCrimmon is the easy winner, outdistancing most Doctors, much less all other companions. This is due to the fact that 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.
 * 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. If Amy Pond stays for a third series, then Rose's record is in jeopardy. Otherwise, Rose's several post-series 2 appearances give her a clear advantage.
 * 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 as of 2011 is the only travelling companion 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.
 * If the Doctor's TARDIS is considered as a companion (see below), she tops all categories, having been a constant in the series since 23rd November 1963.

The Doctor's TARDIS as companion
In the wake of the revelations made in The Doctor's Wife - not the least of which being the confirmation (long implied and assumed) that the TARDIS, if not itself sentient, was controlled by a sentient consciousness - a school of thought has been established that considers the Doctor's TARDIS to be not simply a travelling vessel, but an actual companion. Although this fact has yet (as of mid-Series 6) to be stated explicitly on screen, it has been referenced in reviews (such as this example), in the Doctor Who Confidential episode "Bigger on the Inside", and by Gaiman and Moffat in the pages of Doctor Who Magazine.