Sally

Ippei meets Sally when he knocks her over while running away after witnessing a girl being raped in the park. When Sally realizes he was turned on by witnessing the girl being raped she uses the knowledge to blackmail him into living with him.

Sally claims she is able to tell when a woman wants to be raped and gives him 5 candidates of women he knows around him. Of course Sally herself does not want to be raped and refuses to allow Ippei to even touch her.

Spoiler

Secretly of course Sally herself wants to be raped. She's training him to be a rapist in hopes that he'll lose his cool one day and rape her.