She used to be a cute little sister who would say things like, "When I grow up, I'm going to marry you!"... Recently, Kotone has started to avoid me. I think she's probably a tsundere. There's no way she'd ever hate me. But now she's in a good mood and talking on the phone... Could it be that she's got a boyfriend?! Before my beloved little sister becomes the property of some random guy, I want you to be her first partner. Let's lose our virginity together.