Zatch Bell!

The story follows a little boy with a book, Zatch Bell, who has lost his memory under unusual circumstances. He is found by a professor who sends him to his son, the bookish Kiyomaro Takamine, without realizing that the boy is no ordinary child. Zatch is actually a demon sent to Earth to fight in a tournament held once every thousand years, the winner of which becomes the new king of all demons. A hundred demons, each with a book, have been sent to Earth, and every one of them must find a human partner in order to fight for the throne.

In the human world the demons cannot use their powers on their own: they need a person able to read the spells in their book. And things are not as simple as they seem, because the book only lets its reader cast spells in step with the demon's growth: the more the demon fights, the stronger it becomes, unlocking new spells. Upon learning all this, Kiyomaro decides to help the little demon become king, because Zatch has resolved to become a kind king who will put an end to these brutal battles.