Merlin

Merlin was a legendary wizard.

In the primary universe, the Merlin was a title. As part of one of his schemes, became Arthur's next Merlin in one of his plans. (PROSE: The Creation of Camelot)

The Fourth Doctor met a High Evolutionary known as Merlin the Wise. (COMIC: The Neutron Knights, COMIC: The Tides of Time)

In a parallel universe, an incarnation of the Doctor posed as Merlin. (TV: Battlefield) He aided in King Arthur's birth (PROSE: One Fateful Knight) and 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, and the Seventh Doctor identified the note from Merlin as being written in his own handwriting. (TV: Battlefield)

Most sources agreed that this was a future, red-haired incarnation of the Doctor, (PROSE: Battlefield, Happy Endings, et al) but by one account it was actually the Eighth Doctor, and the prior Merlin of that reality had been only disguised as the Doctor. (PROSE: One Fateful Knight)

Another time, whilst stuck in the otherworld of Avalon, the Eighth Doctor attempted to pass himself off as Merlin to Queen Mab. (PROSE: The Shadows of Avalon)

The Tenth Doctor later claimed that he was once called Merlin by King Arthur. (COMIC: Fugitive)

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.
 * The prologue in Marc Platt's novelisation of Battlefield describes Merlin as having "a head of unruly red hair", wearing a "tatty embroidered Afghan coat", and a "floppy hat of brown felt" with a "saffron Katmandu bandana".