Lancelot

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

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

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)