Lancelot

Sir Lancelot was a knight in the service of King Arthur in Camelot.

As a real person
According to one account, he and the rest of Camelot existed in the alternate timeline where the universe was destroyed by the Doctor's TARDIS exploding. He had a "faithful pooch" named Esmerelda, and had a crush on Rory Williams, who he invited to Merlin's banquet, in order to get to know him. Rory took care of Lancelot as a nurse, helping him recover from an infected wound, using the Heimlich Maneuver to remove an obstruction when he choked at Merlin's feast, and finally cured Lancelot after Merlin poisoned him. Finally, he suggested that King Arthur make Rory a knight, with Merlin's enthusiastic support. (AUDIO: The Once and Future Nurse)

Together the two went on a quest to investigate the death of the Glowing Warrior and became closer friends, (AUDIO: The Glowing Warrior) and returned to Camelot just in time to find that Merlin had taken over. Lancelot watched as Rory helped Guinevere lead the army against Merlin while Arthur was injured, and was present when Arthur defeated the wizard. Rory then sadly chose to leave Camelot and bid Lancelot a fond goodbye. (AUDIO: The Last King of Camelot)

The Fourth Doctor claimed to have taught Lancelot how to use a sword. (PROSE: Wolfsbane)

As fiction
Lancelot was the subject of several works of fiction. The Second Doctor summoned Sir Lancelot in his mental battle with the Master of the Land. (TV: The Mind Robber) Indeed, Tasha Williams believed Lancelot to be entirely fictional, whether or not there was some truth to the basic story of Camelot and King Arthur; she claimed that Lancelot had been a relatively late invention, added to the Arthurian mythos by French writers. She was baffled when Ganeida, the sister of Merlin, personally told her that she had actually met Lancelot, who was "a jerk". (PROSE: A Honeycomb of Souls)

The Thirteenth Doctor remembered telling a knock-knock joke to King Harold which referenced a fictional knight seemingly named in reference to Lancelot, and which she repeated to Harold Wyntonia: "What do you call a knight who always cheats on his tests? Sir Glancelot." (COMIC: Mistress of Chaos)

One of King Arthur's knights in a psychoscaope within the Dalek Dome in 2323 was named Lancer Lotte. (COMIC: )

Behind the scenes

 * The Tenth Doctor's mention in PROSE: Legends of Camelot that Lancelot resembles Ian Chesterton may be a reference to the 1956 ITV series The Adventures of Sir Lancelot, where William Russell portrayed the titular role.