He will lead the devils back to their rightful place as rulers of the world. Humanity is headed towards doom and he knew it. In his devil form, Satan is androgynous, possessing the genitals of both man and woman. Satan used his powerful psychic abilities on himself to keep his devil persona asleep until he was needed to begin the Apocalypse.
Spoilers for the original Devilman:
Satan took the form of a human to observe humanity better. Satan and Ryo are one and the same.