Zatch Bell!

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. 

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Takamine Kiyomaro is a junior high school student who thinks he’s too smart for school, to the disappointment of his mother. On his 14th birthday a surprise literally crashes through Kiyomaro’s window, in the form of… a small naked child?! His name is Gash Bell and his goal is simple: be a mentor for Kiyomaro, at the request of Kiyomaro’s departed father. In return, Kiyomaro must help discover the secrets behind Gash Bell’s amnesia and his past, with only a red book as a clue. As if things weren’t crazy enough, Kiyomaro soon discovers that Gash Bell is a demon -- one of many on Earth -- who can be forced back to the demon world if its corresponding book is burned. Armed with the power of the red book, Kiyomaro must now help Gash Bell defend himself against other demons who want to vanquish him from the Earth, lest his newfound friend vanish forever!