Stellula Eques Codex ~Tasogare no Himekishi~

Akira is an average guy. But one day, Izayoi, his classmate, suddenly seduces him and they have sex... After having sex, big white wings come out of Izayoi's back and she transforms herself into a soldier called Stellula Eques. An evil group also appears and Akira is attacked by them. Izayoi and other soldiers know about their conspiracy and try to stop it. What exactly is Akira's power?! What is Stellula Eques' role?! What will happen to them in the end!?