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 (novel)), 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)

History
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 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.) At any rate, this Merlin had the Doctor's handwriting despite the Seventh Doctor having no memory of having been him yet. Merlin fought the sorceress Morgaine until she imprisoned him.

He 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) However, the Eighth Doctor later came to believe that the Merlin who'd left instructions for the Seventh Doctor was not actually one of his future incarnations but was merely another Time Lord posing as the Doctor, the Doctor manipulating events so that this other Time Lord was trapped by Morgaine in his place while he left the various clues he remembered finding for himself while mitigating the more negative aspects of his influence on Camelot. (PROSE: One Fateful Knight)

1965 Los Angeles was home to many magicians and conjurers. One of them went by the name of "Mervin Merlin"; he had a bushy mustache 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)

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

Behind the scenes

 * In 2008, Julie Gardner became producer of the BBC Wales production, a reimagining of the Merlin and King Arthur legend, and which featured numerous Doctor Who and Torchwood cast and crew members, including Colin Morgan, Anthony Head, Angel Coulby and Richard Wilson in starring roles. Guest stars included Pauline Collins.
 * Daniel Brocklebank played a young version of Merlin in the 1998 miniseries.