Zatch Bell!(2003)

Takamine Kiyomaro, a depressed don't-care-about-the-world guy, was suddenly given a little demon named Gash Bell to take care of. Little does he know that Gash is embroiled into an intense fight to see who is the ruler of the demon world. All of the demons have to pick a master on Earth and duke it out with other demons until one survives. Needless to say, Kiyomaro becomes Gash's master, and through their many battles, Kiyomaro learns the importance of friendship and courage.