Doctor Who wouldn't work as well as it would if it weren't for the companions -- the ordinary folks that travel with the Doctor and share in his adventures. Practically, with the companions present for audience identification, the Doctor is free to be more peculiar and alien; and increasingly the companions have been serving a vital role in helping the Doctor understand himself as he adjusts to becoming the last of the Time Lords.
(* = voice only.)
44. Arthur Darvill as Rory Williams
Amy's boyfriend and, it later turns out, fiancé, first encountered in "The Eleventh Hour" and later brought on board the TARDIS by the Doctor after he becomes concerned that Amy is becoming too attached to him. A nurse hoping to become a doctor, Rory initially seems timid but perseveres in trying to uncover why coma patients are speaking, and later displays dogged determination. Eleventh Doctor. Stories as a companion include: "The Vampires of Venice", "Amy's Choice", "The Hungry Earth", "Cold Blood".
43. Karen Gillan as Amy Pond
Amelia first encounters the Eleventh Doctor as a serious, self-possessed, and remarkably insightful little girl who's troubled by a surreal crack in her bedroom wall. The Doctor later returns, 12 years after he'd promised to, to find Amy, a serious, self-possessed, and remarkably insightful young woman whose spent the last dozen years obsessing on her childhood visitor. Eleventh Doctor. Stories as a companion include: Seasons 5 and 6 (ongoing).
42. Bernard Cribbins as Wilfred Mott
First encountered as a crusty newsagent in an abandoned London during "Voyage of the Damned," Wilf turns out to be Donna's stargazing grandfather and helps the Doctor conceal his and Donna's activities from Donna's mother. Troubled by visions of a strange woman and nightmares about the Master, Wilf seeks out the Doctor, who's unnerved that Wilf could do so so easily. The Tenth Doctor accepts death to save Wilf from a lethal dose of nuclear energy, even though he is not ready to die. Tenth Doctor. Stories as a companion include: "The End of Time".
41. Lindsay Duncan as Adelaide Brooke
Stern, efficient commander of Bowie Station, the first human colony on Mars. Gradually realizing the Doctor has future knowledge of the base's destruction, she at first demand his help, then accepts his refusal. When the Doctor violates what he believes in to save her after all, she is appalled by his arrogance. She puts the timeline right by killing herself, causing the Doctor to repent of his hubris. Tenth Doctor. Stories as a companion include: "The Waters of Mars".
40. Michelle Ryan as Lady Christina de Souza
A noblewoman and master thief who is aboard a city bus that drives through a wormhole onto a deserted desert world. Christina immediately takes charge of the situation, pairing herself with the Doctor to resolve the crisis, and asks eagerly to join the Doctor after all's well. He declines, refusing to endanger or lose any more friends. Tenth Doctor. Stories as a companion include: "Planet of the Dead".
39. David Morrissey as Jackson Lake
A Victorian man who, during an attack by Cybermen, becomes imprinted with the Cybermen's infostamps containing all their knowledge of the Doctor. The horror of seeing his wife killed and his young son taken away causes him to enter a fugue state in which he thinks he is the Doctor, severely confusing the Tenth Doctor when they meet and initially causing him to think that Lake is an amnesiac future incarnation of himself. Tenth Doctor. Stories as a companion include: "The Next Doctor".
38. Kylie Minogue as Astrid Peth
Astrid is a starry-eyed waitress aboard the spaceship Titanic. As the Doctor works to reveal a plot to destroy the ship, she becomes attached to him, and sacrifices herself to save the Doctor's life. Tenth Doctor. Stories as a companion include: "Voyage of the Damned".
37. Freema Agyeman as Martha Jones
A medical student whose the Doctor enlists when an entire London hospital is stolen and taken to the Moon. Devoted to the Doctor but hurt by his fixation on the now-departed Rose, Martha toughens up during their time together and leaves the TARDIS of her own accord. Newly self-empowered, she joins UNIT's medical team, and also works temporarily with Torchwood. Tenth Doctor. Stories as a companion include: "Smith and Jones", "The Shakespeare Code", "Gridlock", "Daleks in Manhattan", "Evolution of the Daleks", "The Lazarus Experiment", "42", "Human Nature", "The Family of Blood", "Blink", "Utopia", "The Sound of Drums", "Last of the Time Lords", "The Stolen Earth", "Journey's End".
36. Catherine Tate as Donna Noble
A career temp with a cavalier, gossipy attitude, Donna immediately perceives that the Doctor needs someone with him to stop him from going to far. Donna later seeks him out successfully and joins him. Touching the Doctor's severed hand during an abortive regeneration causes her to take on a Time Lord mind, which her human brain cannot support; to survive she must regress to what she was before she met him. Tenth Doctor. Stories as a companion include: "The Runaway Bride", "Partners in Crime", "The Fires of Pompeii", "Planet of the Ood", "The Sontaran Stratagem", "The Poison Sky", "The Doctor's Daughter", "The Unicorn and the Wasp", "Silence in the Library", "Forest of the Dead", "Midnight", "Turn Left", "The Stolen Earth", "Journey's End".
35. Noel Clarke as Mickey Smith
Rose's boyfriend in "Rose," he's initially unable to cope with the same bizarre events Rose finds invigorating, and in subsequent encounters sees the Doctor as a threat to his relationship with Rose. Nonetheless, he serves as their contact and agent on Earth, and after a few adventures of his own stays behind on an alternate Earth to fight the Cybermen, later returning to our Earth when they do. Eventually becomes a freelance mercenary against alien invaders. Ninth, Tenth Doctors. Stories as a companion include: "School Reunion", "The Girl in the Fireplace", "Rise of the Cybermen", "The Age of Steel", "Army of Ghosts", "Doomsday", "Journey's End".