Muldwych

Muldwych spent a thousand years stranded on Mount Kukoeuk on Antýkhon. He assisted Bernice Summerfield and Ace with the Doctor's TARDIS. (PROSE: Birthright)

He gave the Seventh Doctor time rings for Bernice and Jason Kane's wedding.

There is some suggestion that he was a future or alternate incarnation of the Doctor, possibly Merlin. (PROSE: Happy Endings)