Ryou Asuka is the best friend of Akira Fudo, the protagonist of Devilman. Though the two were separated after Akira switched schools, they were reunited when Ryou sought Akira’s help to stop the demons which had begun to reappear in the world. In order to better their chances of success, Ryou suggests the two of them fuse themselves to become a devilman—heart of a human, but the body and power of a demon—though only Akira is successful. As Devilman, Akira works with Ryou to fight the increasing onslaught of demons.
It is revealed at the end of the manga that Ryou Asuka is actually Satan himself. In order to take back the world from God who, in favor of humans, had banished demons from the world, Satan wipes his own memories and takes the role of a human boy, Ryou Asuka, to discover humanity’s weaknesses. In the process of doing so, however, Satan fell prey to one of humanities’ greatest weaknesses; he fell in love with a human, his friend Akira Fudo.