Konjiki no Gash Bell!!

Kiyomaro Takamine, genius middle schooler, is greeted one morning by Gash Bell, a boy with a mysterious red book. Tasked by his father to decipher it, Kiyomaro soon learns Gash is no ordinary boy. Able to shoot lightning from his mouth at the chant of a word, Gash is a demon child—one apart of a war that takes place every 1000 years to decide the Demon King. With Kiyomaro being the only one who can read Gash's book, the duty of partner is thrust on him in this exciting tale of friendship, determination, and melons...? (Source: MyanimeList)