Kiyomaro, or Kiyo as he's commonly called in the English version, was a regular 14-year-old genius until his father found Zatch Bell on an archaeological trip. Zatch was dying, and Kiyo's father saved him. Feeling indebted, Zatch Bell said he would do anything to repay the favor, thus Professor Takamine asks Zatch to go and help his arrogant son Kiyo learn to make friends.

Before Kiyo was given Zatch, he really didn't have any friends because everyone thought that Kiyo acted like he was too good for them. Zatch started operation "Hero of Justice" and made Kiyo belive in himself.

Now Zatch and Kiyo work together to defeat other Mamodos and to win the battle to become king of the Mamodo world. Kiyo reads spells out of Zatch's book, which is also Zatch's link to Kiyo's world.