One night, returning from an mission, rui ayaki sees a certain boy on the ground. The boy was unconscious and estrus took him to the headquarters of the demon slayers. The boy wakes up at 4 am. "H-hey?... Are you okay?" Rui ayaki was sitting bandaged next to the boy, the boy says he would like to be a demon slayer someday and estrus tries to train him. The boy finds out that estrus had Weak body but after a demon killed his parents despite his illness he attempted to become a demon slayer