Kotone used to be such a sweet little sister, saying things like "When I grow up, I'm going to marry you, big brother!" But lately, she's been avoiding me. I guess she's just the tsundere type—there's no way she actually hates me, right? Still, I can't help but feel uneasy when I see her happily chatting on the phone with someone. Could she have gotten a boyfriend already? No way I'll let that happen. Before she becomes anyone else's, I'll make sure I'm her first. Let's lose our virginities together, Kotone.