Satan is the King of Demons and the eternal adversary of God in Go Nagai's Devilman franchise. Apparently working behind the scenes throughout the events of the series, it turns out that Satan has taken on the form of Akira Fudo's best friend Ryo Asuka.
Ryo Asuka and Akira Fudo were once childhood friends, but parted ways due to unknown circumstances. When they met again while Akira was in high school, Ryo revealed to Akira a terrible secret left behind by his father. Professor Asuka had discovered the existence of demons in the human world and had become possessed by such a demon, but committed suicide in order to prevent the monster from controlling him. After studying his father's notes, Ryo sought out Akira with the intention of sharing his father's horrific legacy with him.
It turned out that Ryo's father had intentionally tried to merge with a demon in order to gain its power, so that he could fight against demons on equal terms and protect the world. Ryo learned that it was indeed possible for a human to retain their consciousness after melding with a demon: someone with a pure heart and noble soul would be capable of taking on a demon's strength and abilities without their mind being consumed by the demon's malevolent influence. Thus, Ryo convinced Akira into joining him in a crusade to rid the Earth of demons and they engaged in a modernized version of the Black Sabbath ritual in order to merge with demonic entities. During the Sabbath, Akira merged with the demon warlord Amon and became Devilman, but Ryo was unable to meld with a demon at all and was almost killed during the fight between Devilman and a horde of monsters.
Ryo would eventually recover from his injuries and try to help Akira as best he could in the war against the demons, but in time the armies of Hell would increase their attacks upon the Earth and Ryo would encounter the demon known as Psycho Jenny. Psycho Jenny revealed that Ryo was not who he appeared or thought himself to be: the real Ryo Asuka had died some time ago and that he was, in fact, Satan, sent out to infiltrate the human world. He was hypnotized by Psycho Jenny so as to perfect his disguise, but during the time he spent with Akira, his friendship grew into love.
Akira would not return Satan's love for him and the two fought a terrible war that lasted over twenty years. By the time the fighting ceased, the Earth had been all but destroyed. In the end, Satan tells Akira that God saw the demons had evolved into what he had considered ugly, so Satan rebelled against their father and the demons were sealed in ice by and with Satan. When the demons returned, humans had taken over so they tried to wipe them out. Satan tells Akira they regret their actions since wiping out humans was just as bad as what God tried to do to the demons and looks over to Akira to find that he is dead (all that is left of him is the waist up) and Satan cries over killing Akira.
Ryo appears as a tall, pale male with somewhat feminine features. He has blue eyes and blonde hair. In his true form as Satan, he is actually a hermaphrodite with no exact gender. Satan is far from the horned monster that so many imagine him to be and appears as an angel with white skin, blonde hair and twelve feathery wings and is stunningly attractive.
The extent of Ryo's/Satan's abilities is unknown, but he is an accomplished fighter, can rapidly recover from any injury, can fly, teleport, and sense the presence of other demons nearby.