Ranthru, from Ultima Underworld
Ranthru is a mage of the Seers of the Moonstone.
The Avatar met him in Ultima Underworld I on the sixth level of the Abyss. Ranthru first asked, if the Avatar was in league with the evil mage Vilus. After getting a negative answer, Ranthru told the story how Vilus turned evil and left for the north-eastern part of the level. They had no idea, if he was still alive (he wasn't).
Ranthru added that he'd taken an important book, "On the Properties of Runestones", with him and that he wanted it back, promising training as a reward. The Avatar found the book and returned it. Ranthru held his promise, and his training increased the Avatar's casting skill.
Later, Garamon rescued him when the volcano erupted.