Zatch Bell! Vol 1

Zatch Bell! by Makoto Raiku. A Shonen manga rated Teen, published in English by Viz Media.

Kiyo Takamine is a genius middle schooler with no friends, until his father sends him Zatch, a strange little boy who shoots lightning from his mouth. Zatch is one of a hundred mamodo battling to become king of their world, and he needs Kiyo's help to win.