Kyoko is a character from Saint Seiya, and the Equuleus Saintia in XX Century.
Shoko, Kyoko's sister, was cursed to become the host of Eris, a god that will cause war around the world. Kyoko always was mature, and tried to protect Shoko from dangers. At night, Atë tried to possess Shoko's body, and the love of Kyoko and his goal to protect his younger sister, allowed Kyoko access the Dark Eden (which only Shoko and Atë, and the Dryads, Eris and the host can access) and saves Shoko; after all, Milo saves Shoko and Kyoko knew about her course; so, Kyoko followed Milo's advice and goes to Graad Foundation to become a saintia, to protect Shoko. Shoko always misses Kyoko. She was sent with Mii, Xiaoling, Erda and Katya to training in another country.
During the training, Kyoko met Rigel, who fell in love with her. After the graduation, Kyoko came back to Japan to protect Shoko and Saori (protecting Saori is the mission of the saintias). But, Atë attacks again and finds with her again. But, Kyoko doesn't get away, and fights, and because of this, Atë defeats Kyoko. After trying possess to Shoko, Kyoko saves her again, and receives Eris' soul in her place. Because of this (Kyoko is the wrong host), Eris need to sleep in the body in a coma, to get total control of Kyoko's body.
Eris woke up before the right time, and tried to kill Saori, but because the process was not completed (because Eris woke up too early), Kyoko could control her body to disturb Eris, and Atë saves her master.
In the Eris sanctuary, Shoko and Saori try saving Kyoko at the last time, but Eris' soul disturbs them; to stop the complete possession, Kyoko holds Eris' soul, and merges with it, allowing Milo to kill her body.
Kyoko revives as an evil being, because her soul merged with Eris' soul, and become an evil woman.