She's loved by the protagonist's sister, but she's a total yandere.

Today, the protagonist is working hard on building his harem. I've been forced to be his friend since we were kids just because our houses are close. No matter what I do, I'm compared to him and met with exasperation every day. That said, his personality isn't rotten at all; he's the definition of a social butterfly. That's exactly why I, who have no standout qualities, end up standing out even more. I was just like that until one day, I received a love letter. Upon opening it, it read, "I love you. I'll be waiting behind the school building after class today." For once, I got my hopes up. —But reality is cruel. The person waiting for me said, "Eh, who are you?" It seems I and my confession partner accidentally called each other out from the wrong shoe lockers. The next day, another love letter was in my shoe locker. The message read, "I know everything about you."