Nystul, from Ultima Underworld II
Nystul first appeared in Ultima VI, where his vision saved the Avatar from being sacrificed. He helped the Avatar with the Book of Prophecies by directing the hero to Mariah, and helped with the Orb of the Moons by directing the hero to Lord British. Nystul was also present when Lord British and Lord Draxinusom met for the first time at the Shrine of the Codex.
200 years later, at the time of Ultima VII, Nystul was also a victim of the disturbed Ether, having become clumsy and quite senile (Spark even thought at first he was the jester). Thankfully, with the destruction of the Guardian's generator, his sanity was restored, and he helped the Avatar by selling him spells and reagents. After the destruction of the Black Gate, he invested a lot of work into restoring the teaching of magic to Britannia.
Nystul was essential for the plot of Ultima Underworld II. He researched methods of breaking the Blackrock Dome, and gave the Avatar much good advice, offering training in the mystical arts as well. Also, he treated the small Blackrock Gems for the Avatar, and later decyphered the Spellbook of Mors Gotha. Together with the Avatar, he managed to free the castle from the Guardian's grip. He cast the required spell from the Mors Gotha's spellbook, while the Avatar used the Horn of Praecor Loth to destroy the dome.