Imouto no Tomodachi no Bijin Yankee JK: Sekenshi Suru Sugite Sewa wo Yaiteitara Horeraremashita

A college student named Tsukasa, who has never had a romantic relationship, finds his room frequently occupied by his sister and her delinquent friends. One day, he discovers Erika, the most attractive of the group, asleep in his bed. She explains she is tired due to her period. Tsukasa notices she is not fully dressed and decides to teach her some basic social skills. Erika, who begins to develop feelings for him, openly expresses her affection. As Tsukasa becomes more involved with her, he notices her making efforts to improve herself. Their relationship unfolds as they navigate the complexities of their growing emotions.