Fintor, from Ultima Underworld
Fintor is a mage imprisoned by Tyball.
After the Avatar freed him in Ultima Underworld I from his cell on the seventh level of the Abyss, talking with him revealed that the imprisonment had made him paranoid and mad. However, despite trying to not reveal it, Fintor revealed in his ramblings the secret of hidden doors in the crypts and the existence of something important at the bottom of the chasm of fire.
Later, Garamon rescued him when the volcano erupted.