One-Eyed One

The One-Eyed One was an "ancient" one-eyed incarnation of the Hermit who, by some accounts, had been the mentor of the young First Doctor. He taught the Doctor about the Other. (PROSE:  Revelation)

Once, the Hermit told the Doctor to fast for three days and three nights to make supplications to the Menti Celesti. (PROSE:  Revelation) Towards the end of the Doctor's first life, the Hermit helped him prepare for his upcoming regeneration by telling him the story of Penultimate Evil and returning to him his Five Hundred Year Diary. (PROSE: The Three Paths)

The Hermit's tutelage of the Doctor had been part of a wider game that the Hermit played in the universe, of which the Seventh Doctor's agendas eventually played a part. While the Doctor was fighting the Timewyrm, the one-eyed Hermit soundlessly appeared inside Saul and ordered the Doctor to "make the final move". (PROSE:  Revelation)

Behind the scenes

 * A deleted scene of No Future would have named this incarnation of the Hermit as the "One-Eyed One".
 * The Odinic or Myrddin-esque nature of the One-Eyed One reflects Paul Cornell's general interest with the threefold death. In Cornell's novels, the Doctor's status as a "raven" possibly reflects his relation with the Odin-like figure.