Toushin Devilman

The ruler of the demon world Zenon gives the order to kill the humans. One of the four heavenly demons Amon however did not view humanity to be as weak as the others believed. This caused unease within the rest of the ranks and Amon escaped them before encountering a junior high student named Kei Kamishiro. The two made an agreement to work together by fusing together as the new identity Devilman.

(Source: Mangadex)