Over the course of its many years on television, the long-running British science fiction television series Doctor Who has not only seen changes in the actors to play the Doctor, but in the supporting cast as well.
The Doctor is usually accompanied in his travels by one to three companions (sometimes called assistants). These characters provide a surrogate with whom the audience can identify, and further the story by asking questions and getting into trouble, (similar to Dr. Watson in the Sherlock Holmes mysteries.) The Doctor regularly gains new companions and loses old ones; sometimes they return home, or find new causes on worlds they have visited. A few of the companions have died during the course of the series.
This list includes characters who appear to be human and who are not known to be anything other than human, even if having originated extra-terrestrially.
Lethbridge-Stewart appeared as a regular in Seasons 7 and 8 and more sporadically in many other episodes. Nicholas Courtney, along with his role as Bret Vyon in The Daleks' Master Plan, his appearance in the charity special Dimensions in Time and his participation in the Eighth Doctor audio play Minuet in Hell, has the distinction of having acted with every screen Doctor before the Ninth and also the Tenth (although in adventures before actor David Tennant was cast as the Doctor).
The Inquisitor and The Valeyard appeared in every episode of Season 23, a season that comprised just one story, (albeit split into four segments), The Trial of a Time Lord.
Mickey Smith was a significant recurring character in the 2005 series, prior to briefly becoming a companion in the 2006 series. Similarly, Jackie Tyler appeared in many episodes of the 2005 and 2006 series; in the episodes "Army of Ghosts" and "Doomsday", she briefly travels in the TARDIS and acts like a companion, although she is not generally considered one.
The Master appeared as a regular in Season 8 and has returned numerous times in subsequent seasons and the television movie.
The Doctor Who comics, novels and audio dramas have created companions, villains and supporting characters of their own. Some of these originated in one medium and later appeared in another. The lists below indicate where a character has appeared.
^Miss Dexter appears in the Tenth Doctor episodes "42" in which she is credited simply as 'Sinister Woman', and "The Sound of Drums" in which her name is given in the credits.
^Queen Elizabeth I is referred to in "The End of Time" in which the Tenth Doctor explains that he married her (the wedding was later filmed for "The Day of the Doctor"), "The Beast Below" in which her far future successor, Queen Elizabeth X chides the Eleventh Doctor about the relationship, and "The Wedding of River Song" in which the Eleventh Doctor notes that he could revisit her before their wedding (which he later does in "The Day of the Doctor"). The then-Lady Elizabeth is off-camera in "The Power of Three" in which Amy Pond becomes the accidental wife of Henry VIII, thus making Lady Elizabeth her step-daughter and the step-sister of the Doctor's other known wife, River Song. Then-Lady Elizabeth is also off-camera with her sister Queen Mary, in Lost in Time, their approach spelling doom for the PrétendreQueen Jane.
^Queen Elizabeth II is also present but unseen in the Olympic Stadium at the end of the Tenth Doctor episode "Fear Her" and in the Eleventh Doctor mini-episode "Good as Gold", both of which take place with the torch's arrival, and thus after Her Majesty's entrance and formal opening of the games; and is at an off-camera party outside of the TARDIS in the Eleventh Doctor mini-episode, "Bad Night" (the Queen is unidentified in the mini-episode itself, but is revealed as "Liz Two" in the associated behind-the-scenes featurette.
^An alternate universe Queen Elizabeth II is said to have been executed prior to the events of the Third Doctor serial Inferno, and another alternate universe ER II is killed in the Tenth Doctor episode "Turn Left", in the Doctor-less version of the events of "Voyage of the Damned".
^Episode 1 of the Seventh Doctor serial, Remembrance of the Daleks, opens with a voice-over excerpt from John F. Kennedy's American University speech of 10 June 1963 (165 days prior to the episode's 22 November 1963 setting); the episode's cliffhanger end takes place in the early evening of 22 November, at or about the moment of Kennedy's assassination.
'^Additionally, Kennedy's assassination - which occurred on the day before Doctor Who premièred with "An Unearthly Child" (which took place one or two days before the assassination, as indicated by the presence of the French Revolution book in Remembrance of the Daleks) - is addressed in Silver Nemesis, "Dimensions in Time", "Rose", "The Cambridge Spy", and "Let's Kill Hitler". His assassination by Lee Harvey Oswald occurs off-screen during the events of Remembrance of the Daleks (at or about the time of episode 1's cliff-hanger or the beginning of episode 2), and a subtle reference is made in "Asylum of the Daleks" in which the two brief survivors of the starship Alaskas crash are named Harvey and Oswin Oswald.
^Lytton's forename is not provided in either of his television serials. It is provided as Gustave in the novelisation of Attack of the Cybermen and the Doctor Who Magazine story, "Mistaken Identity".
^A former soldier cum mathematics teacher at Coal Hill School, Danny is the colleague and boyfriend of Clara Oswald. He appears in all Series 8 episodes, excepting "Deep Breath" and "Robot of Sherwood". Despite only small scenes in "Mummy on the Orient Express" and "Flatline", he was credited for both. After appearing in ten episodes (most significantly in "The Caretaker", "In the Forest of the Night", "Dark Water" and "Death in Heaven"), he died in the pre-credit scene of "Dark Water", but he still appeared in the post-credit scenes of the episode, and its follower. The turning point for the finale was when Danny sacrificed himself to save Earth from "Missy" (the Master).