This Synopsis was written by me.
Kiyo Takamine is an apathetic but highly intelligent middle schooler who doesn't fit in very well at school. Most of his classmates are jealous of his genius and annoyed at his stand-offish personality(except Susie who, despite being a total ditz, has a big time crush on Kiyo.) But Kiyo's life takes a major turn when his father, who works at a university in London, sends an extremely hyper-active but good natured boy named Zatch Bell to live with Kiyo. Kiyo's father found Zatch lying unconscious in the woods in London with nothing but a red book with him. However, Zatch has no memories of where he came from, and the words in the red book are written in letters that no one can decipher. Kiyo's father hopes that Zatch's friendliness will rub off on Kiyo and help bring him out of his shell. At first, Kiyo rejects Zatch, but after Zatch risks his life to help Kiyo stand up against a school bully, Kiyo finds out that Zatch is not all he appears to be. Kiyo is suddenly able to read the letters in Zatch's book, and when he reads the strange word "Zakeru," Zatch shoots off a powerful lightning attack from his mouth! As Zatch and Kiyo are confronted by others who also have strange books and powerful attacks, they finally discover what it's all about. Zatch is a "mamodo" from the mamodo world who was sent to Earth along with 99 other mamodos. The mamodos are to team up with a human who can read their spell book and battle against other mamodos in order to determine who will be the next mamodo king. The winner of each mamodo battle must burn the loser's spell book in order to send that mamodo back to the mamodo world. So the last mamodo remaining on Earth will be the new mamodo king. Unfortunately there are many evil, power-hungry mamodos who team up with equally power-hungry humans. There are also mamodos who do not wish to battle and only want to live happily with their human partner. At first, Zatch and Kiyo did not want to be involved in the battle to become mamodo king, but after the gentle mamodo Kolulu was forced to battle against her will, only to have her book burned and thus, she was sent back to the mamodo world and torn apart from the human partner she had grown to love, Zatch and Kiyo made a promise to Kolulu: they would continue to battle and defeat all of the evil mamodos so that Zatch will become the next king; a kind and benevolent king who would see to it that the mamodos would never be forced to fight again.
As Zatch and Kiyo continue to face off against evil mamodos, they discover more about the mamodo world and about Zatch's lost memory, new spells appear in their book, and they even meet a very evil mamodo who looks just like Zatch! But most of all, the strong bond of partnership between them continues to grow. Although most of the bad mamodos they face only want to fight and become king, Zatch and Kiyo also meet other mamodos and their human partners that they're able to become friends with; like pop-star Megumi and her mamodo Tia, charismatic Italian dancer Folgore and his duck-billed mamodo Kyanchome, the orphaned horse mamodo Ponygon, the mysterious Sherrie and her stoic yet powerful mamodo Brago, and an ongoing cast of wild and colorful characters. As the battle to determine the mamodo king goes on, Zatch and Kiyo continue to stand up against the evil ones, and although their battles are harsh, they're determined to end the mamodo battles and make Zatch the kind and caring king that the mamodo world desperately needs.