Count Taffy would be my suggestion. He is a kind of old wise character whose presence can be reached after a strenuous task, so to add to his presence. I want to suggest probably introducing him after passing 5 consecutive levels on a first try. The rewards are gold bars.
I like this concept of one in every ten levels cleared. Introduce after a high number of levels are passed though, as a reward for staying with the game. Cheers.