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.
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.
^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.
^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.