Zato Cero a delinquent who attends Kouh Academy. He's admitting a strange energy which catches Rias' attention, however he's not so cooperative. He wants little to do with anyone and distances everyone that tries to get close to him, why?

Shirou Kazehaya have been working as Kaguya Shinomiya's security detail as long as he remember until he saw another beautiful girl that always in Shinomiya's side, her name was Ai Hayasaka and he have fall in love with her.