The character you're referring to is Anthys' brother, who serves as the acting chairman of the academy. Although he had a minor presence in the first season, his role became more significant in seasons two and three. His name comes from the Japanese term for Venus, which symbolizes Lucifer.
In his youth, Akio and Dios (Spanish word for "God") were apparently one and the same, but he became corrupted and lost his purpose as a "prince." Giv