A bishop from the Barsburg Church, he convinces Teito to take the examination to enter the priesthood. The training he puts Teito through is vigorous and often involves his 'dolls', life-sized puppets he controls with Zaiphon. One of the 07-Ghosts, he is Fest.

After Teito discovers that Castor and Frau are both one of the Seven Ghosts, Castor decides to wipe away Teito's memories of the incident, because their identities must remain a secret. Frau, however, stops him, telling him that if he takes away Teito's memories of when Frau revealed himself as Zehel, Teito will not remember Mikage's last smile (Frau transformed to save him when Mikage was possessed by Ayanami).

His birthday is the 24th of December.