Amon is a massive hulking demon that had great strength, speed and intelligence. In hell, he was known as the god of war. He was infamous among the demon and was the lover of the angelic she demon Sirene, although despite his immense power he still despised the Angel known as Satan.
During a Black Sabbath ritual held by Ryo Asuka (Satan's human form) many demons took control of the partiers there, however, when Amon tried to take over the body of Akira Fudo. However, this had been planned by Ryo and due to Akira's pure and innocent hart Amon's conciousness was suppresed and Akira became Devilman. Then with his new body, he quickly slaughtered all the other demons.
Much later when the reveal of Demons had been broadcast across the entire world, Akira had set up a small army of people like him who were called Devilmen. However thanks to the Demon Psycho Jenny, Ryo had turned back into Satan, who then released footage on to the news of Akira's transformation into Devilman. As soon as he had done this, a riot formed outside the Makimura's home, the place that Akira had been living, the mob broke in and brutally kill Miki Makimura Akira's love and her brother Tare. After finding their bodies, Akira kills the rioters, and from then on, he slowly loses control of his mind and body.
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Some concept art
A short while after the young Devilman Yumi comes to find him along with Miko another Devilman, they find him in an abandoned warehouse were they are suddenly attacked by Seylos and his small Demon armyé Seylos says that he would let Yumi and Miko live if Akira joins him, however at the same time Akira finally surrenders his body over to Amon and destroys the factory in the process. Outside, he finds Yumi unconscious and proceeds to eat her. He then notices Miko, but before being able to kill her, he is attacked by the Demons that accompanied Seylos. However he kills all of them in a matter of minutes before confronting Seylos. The two battle in the air before landing and Amon breaking Seylos' wrists, then he jumps up and slams his fist through Seylos' body killing him.
Amon vs. Devilman
He then finally confronts Satan, but Psycho Jenny uses her powers to reawaken Akira inside Amon. Inside Akira' head, Amon and Devilman fight, but Amon being as powerful as he is quickly gets the upper hand, but when he starts to taunt him over all his friends and families' deaths, Devilman starts to beat back Amon and before rapidly punching him so fast that Amon can no longer stand were he eventually faints and Akira reasserts himself over his body.