Every 100 years, a battle of 100 demon children are sent to Earth to compete in a battle to become king of the Demon World. Each demon is paired with a human partner who is given a spell book of a specific color. The human can cast the demon's spells through their book. If a demon's book is burned, they disappear and return to the Demon World. The last demon remaining becomes the king. The show is oriented around Gash Bell and his human partner, Takamine Kiyomaro.
From Viz:
For his 14th birthday, Kiyo received a strange little boy named Zatch Bell as a bizarre gift from his father. Kiyo soon discovers that there's more to Zatch than his dad ever realized--Zatch is a mamodo child with magic powers that are unleashed by reading spells from a mysterious red book...and there are 99 other kids like Zatch who must fight one another to become the king of the mamodo world!
