Merlin

Merlin was a legendary wizard connected to Arthurian legend.

Merlin's identity was debated; in fact, one account outright stated that "the Merlin" of King Arthur's court was a title rather than a proper name, passed down when the previous Merlin retired or died. (PROSE: The Creation of Camelot) The Doctor was among the individuals who adopted the name at one point or another, (PROSE: Legends of Camelot) in several incarnations including his eighth (PROSE: One Fateful Knight) and a future one (PROSE: Battlefield) who believed himself to be "the last". (AUDIO: Shadow of a Doubt) According to one account, the "original" Merlin was an ancient entity from the universe before universe. (PROSE: Legends of Camelot)

History
According to one account, the original Merlin existed the universe before universe and was distinct from the Doctor. He was also known as the Champion of Neutrality. Seeking to end the battle between the Guardian of Might and the Guardian of Magic before it destroyed all of reality, he proposed that the two Guardians work out their conflict through human puppets unaware of the cosmic forces shaping their destinies: King Arthur and Morgaine. However, the Guardian of Magic kept resetting the game, altering the identity and powers of her avatar, to win at any cost.

To break the cycle, Merlin trapped the two Guardians in the Druse in the Void — and himself with them, locking the three of them in eternal sleep, giving them dreams that the game was still going on. However, because there were cracks in the Druse, this was in fact true, with the three entities' dreams continuing to affect events in the physical world. Merlin thus influenced others, most notably the Doctor, to act as his own avatars in the ongoing "game", arranging events so that they would use the name Merlin and do what was needed to prevent the Guardians from escaping their prison. (PROSE: Legends of Camelot)

The first individual to use the name "Merlin" in Camelot was a Renegade Time Lord distinct from the Doctor, who used the alias when making a deal with Uther Pendragon to allow him to take the appearance of Igerna's lawful husband Gorlois to trick her into sleeping with him. This resulted in the conception of King Arthur. (PROSE: One Fateful Knight)

After the Seventh Doctor discovered that one of his future selves would take on the name "Merlin", (TV: Battlefield) the Eighth Doctor used the alias when assisting in King Arthur's birth. (PROSE: One Fateful Knight) The Tenth Doctor later claimed that he was once called Merlin by King Arthur. (COMIC: Fugitive)

A different, presumably human individual named Blaise became the Merlin. Shortly after his death, one incarnation of the Master arrived in Camelot and became the new Merlin, attempting to manipulate the now-adult Arthur for his own ends. The Fifth Doctor interfered, exposing the Master's schemes and forcing him to flee. Meanwhile, Arthur was inspired to found an order of Knights. (PROSE: The Creation of Camelot)

Most sources agreed that a future, red-haired incarnation of the Doctor then became Merlin, (PROSE: Battlefield, Happy Endings, et al.) although according to another account, the Merlin who returned to King Arthur in his adulthood was still the Eighth Doctor. (PROSE: One Fateful Knight)

One Merlin active during the heyday of King Arthur and his knights was the half-brother of Ganeida, who was a wizard like himself; the two were fond of each other, although they did not grow up together. They were both "half-demon", granting them their magical abilities and an extended life-span. Agamya speculated that the "demon" father of the pair may actually have been an alien of some kind. (PROSE: A Honeycomb of Souls) Bedevere once noted that the Merlin he was familiar with had ginger hair and a "box of tricks". (PROSE: The Door We Forgot) He possessed a magic lockpick. (AUDIO: Broken Bonds)

At any rate, one Merlin had the Doctor's handwriting despite the Seventh Doctor having no memory of having been him yet. Merlin helped Arthur fight the sorceress Morgaine, allegedly until she imprisoned him in "the ice caves"; (TV: Battlefield) however, the Doctor actually escaped this fate because the man Morgaine captured and trapped in the Arctic Desert was actually the other Time Lord using the name, who had used his chameleon circuit to make himself resemble the Doctor in order to frame him for a betrayal of Arthur. (PROSE: One Fateful Knight)

One account depicted a Merlin whose "black magic" included a spell that could transport a man through time, although he had to obey the Laws of Time. He had a long white beard and moustache, and wore a black robe and pointed cap embroidered with star symbols. This Merlin was apparently immortal, surviving into the 20th century, but he had to go into hiding, living as the obscure, elderly owner of a curiosity shop in the Balkans. By then, he had cut his beard shorter, and taken to wearing a fez. (COMIC: Dr. Who's Time Tales) Interestingly, several incarnations of the Doctor were fond of fezzes. (TV: The Day of the Doctor, Kerblam!)

The "Merlin" Doctor left instructions that would help the Seventh Doctor defeat Morgaine in Carbury in 1997. Ancelyn from Arthur's World identified the Seventh Doctor as Merlin, even though he did not recognise his face; thus, the Seventh Doctor essentially succeeded his probable future self as Merlin. (TV: Battlefield)

1965 Los Angeles was home to many magicians and conjurers. One of them went by the name of "Mervin Merlin"; he had a bushy mooustache and wore a classic wizard's hat and cape over a more modern smoking jacket. (COMIC: Who Killed Death?)

A humanoid with a long white beard and pointed ears who called himself Merlin the Wise was encountered by the Fourth Doctor. (COMIC: The Neutron Knights) Merlin sat on a council of the Higher Evolutionaries in the Matrix alongside Rassilon; the Fifth Doctor reacted with surprise upon seeing him in this position. (COMIC: The Tides of Time)

Legacy
An item labelled as the foreskin of Merlin was contained in the Hecht Museum of Unexplained and Occult Material. (COMIC: Project: Longinus)

Behind the scenes

 * Colin Morgan played a young version of Merlin in the BBC Wales series, starring Morgan alongside Anthony Head, Angel Coulby, Richard Wilson, and John Hurt, with Alexander Vlahos, Michelle Ryan, and Lindsay Duncan playing recurring characters. Julie Gardner was an executive producer, and various episodes were directed by Jeremy Webb, Alice Troughton, Justin Molotnikov, Ashley Way, and James Hawes.
 * Daniel Brocklebank played a young version of Merlin in the 1998 miniseries.