Merlin

Merlin was a Time Lord who posed as a magician in a parallel universe.

The Seventh Doctor first learned of Merlin while fighting Merlin's enemy, the sorceress Morgaine, in Carbury in 1997. Ancelyn from Arthur's World identified the Doctor as Merlin, even though he did not recognise his face. The Doctor thought that Ancelyn may have known an alternative or future incarnation of himself. According to Ancelyn, Merlin had a ship like a TARDIS and would be imprisoned by Morgaine. Merlin left a note for the Doctor, which he recognised as his own handwriting. (TV: Battlefield)

The Doctor later met Muldwych, a future incarnation of himself who may have posed as Merlin. (PROSE: Happy Endings)

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 conundrum of Merlin's true identity was apparently resolved when the Eighth Doctor travelled to Arthur's reality to aid in his birth. As the Doctor discovered, he was only the second Merlin. The first was another renegade Time Lord who had given Arthur and his kingdom advanced technology, prompting Morgaine to seek an alliance with them. This other Merlin, disguised as the Doctor, was imprisoned in the Arctic by Morgaine after she was temporarily banished to a desert by the Doctor. This left the Doctor to set up the events that were later experienced by his seventh incarnation. (PROSE: One Fateful Knight)

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

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

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

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. Although there was some initial fan speculation if it would be possible to reconcile the new series with the Merlin presented in Doctor Who, the new concept of the character does not seem to allow for this.
 * 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"., no Doctor depicted on screen has had red hair.