Take Jacob the Sandshrew. Jacob is very timid ans as such, he is unwilling to stay in battle without protecting himself. Through traveling and battling with your party, he begins to gain some confidence. Once he feels he is ready to prove himself, he must solo your rival at the Pokemon Tower (minus eevee of course) in order to evolve. After winning, his confidence is through the roof and he wants to lead your party for the rest of the game. He also picks up a personal vendetta against your rival's Sandslash, and must solo it any time you encounter him again.
Just need one more for my Emerald scramble.
EDIT: Ooh...PSDuckie, I didn't think about that lol. Looks like Jacob will be soloing Misty unless someone saves you.