Amon

Demon Lord of War, Amon is one of the most powerful demons in all of Hell. His destruction on Earth and his affair with Sirene were known throughout Demon Realm. His failed attempt to possess Akira resulted in the birth of Devilman.

[Spoiler] During Miki's murder he is able to gain back control and goes on a killing spree, eventually killing humans, demons and attacking Satan. He is then challenged by Akira for possession of his body. Amon had the upper-hand but eventually was defeated by Devilman. [/Spoiler]