Тайный роман, скрытый от подруги - младшая сестра моей подруги симпатична, когда она соблазняет меня - Iichika Kasagi

Ichika is an honor student and quiet, but she actually loves to do naughty things. The man she falls in love with is her older sister's boyfriend... She finally seduces him when she gets to be alone.