Ryo Asuka was the human alias of the Angel known as Satan. His identy was assumed after his assumed death, thus never physically appearing in the films.
Ryo was a handsome blond boy with piercing blue eyes.
Ryo was the son of the esteemed archeologist Professor Asuka. At some point preceeding the events of Akira Fudo's transformation, he is killed and his identity is taken over by Satan.