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.
SpoilerCastor's original name was Xinglu Hausen, a descendant of the 6th District's God House. The jobs of the Hausen House was to train assassins to deal with dissenters to the Empire. He grew up with a father who treated him strictly, and his mother was the only person who spoiled him. His mother's love influenced him to create dolls in her image, claiming that he would make a mother just for him because his real mother belonged to his father.
On one assassination mission, he heard beautiful singing coming from a noel mermaid locked in a cage. Just as the mermaid, Razette, became attached to Xinglu, Xinglu was taken aback by the beauty of her singing voice and rescued her. And she, in turn, opened his heart through her song and taught him how to smile. This changed his influences in doll crafting--he now created Razette dolls instead of mother dolls. Xinglu even considered traveling the world as a puppetmaster, with Razette singing the songs to his shows.
But after the Raggs war, because the Hausen House supported of the Raggs Kingdom, they were ordered to hand over Xinglu's head. Xinglu's father, desperate not to lose his son and successor, insisted that Razette change her face so she can be offered as a sacrificial lamb. Although Razette understood and was ready to comply, Xinglu stepped in the way of his father's sword and was fatally wounded. In his dying moments, he saw only the spirit of Fest floating above him and has since then become one of the Seven Ghosts.
Seiyuu: Chiba Susumu