Gash Bell and Kiyomaro Takamine originate from the manga Konjiki no Gash!! by Raiku Makoto.  An anime version was later made and released under the dub title Zatch Bell.

Takamine Kiyomaro was a social misfit. He was so much smarter than everyone in his class that he finally stopped going to school.  The rest of his life wasn't much more meaningful, either.

Enter Gash Bell...through Kiyomaro's open window.

Gash Bell was found in the woods by Kiyomaro's father and was given to Kiyomaro as a gift. Kiyomaro wasn't terribly pleased with this as Gash really got on his nerves. But Gash  is no ordinary boy, as Kiyomaro quickly learned. When he reads spells from the red book that Gash came with, the small boy unleashes amazing lightning based powers.

Gash is from another world and was sent to Earth to participate in the battle to become king. Where he was from, Gash is regarded as a weakling and he is often sought out by the other demons from his world because they expect him to be easy to defeat.

But when Kiyomaro and Gash became closer, they both changed drastically. Kiyomaro learned to take initiative with his life and Gash started getting stronger. The two of them might have a chance after all.