Rigel is the Orion Saint in XX Century. His name is a tribute of Rigel, a star of his constellation.
Rigel met Kyoko in the training, and gets a crush on she. After that, he decided protect she, and after Eris possess her body, he become a Eris servant, because he know that to enemies of Eris kill that godess, needs to kill Kyoko, so, he protect Eris life and this is the meaning of a servant.
But, after Eris defeat and Kyoko become a evil person, Rigel did not served Eris and served Kyoko, because always was his goal (protect Kyoko).