Main character shows affection for his classmate for a long time and finally starts dating her.

But she has a secret. Her twin brother lives inside her from birth. During day she can fully control herself, but at night brother's will dominates. As of late, she starts doing strange things at night including having forced sex. Her control periods start to shorten as well. Protagonist vows to make an effort to restore her integrity. However...

[From vndbreview]