The best friend of Akira Fudo, the main character of Nagai Go's classical horror manga Devilman.
As Akira is the strength, Ryo is the brain. His father discovered the existence of demons in our world and decided to fight them. He asked his friend Akira for help. Ryo knew that to be able to beat a demon, one had to become a demon. He and Akira went through a ritual to fuse with demons, but in the end, only Akira was able to do so and became Devilman, leaving Ryo remain as a normal human. Or so they thought.
Ryo's true identity was much more horrible, much more terrifying than he or Akira could've ever imagined. Ryo turned out to be Satan himself, who sought to destroy the world. Satan is depicted as a beautiful hermaphroditic Seraphim angel with twelve wings. However, it is later revealed by Zenon that Satan is in fact in love with Akira. The Devil's love for Akira was developed through the human relationship between Akira and Ryo.