Milenus, from Ultima Underworld II
Milenus was once the most trusted advisor on military matters for his regent. However, he had seen through the Guardian and understood that the Goblins of Tarna effectively had become his slaves. He had no big love for Bishop and his resistance, but understood that Goblins and Humans could only live in peace together, if the Guardian's hold on Tarna was broken, so he became a spy.
However, prior to Ultima Underworld II, Milenus was discovered and thrown into the Prison Tower, where the Avatar freed him after Garg killed all the other Goblins. Milenus told the Avatar his story and added bitterly that while the Avatar is seen as a hero, he would be seen as a traitor. Then he left to go back to his fight against the Guardian.