Akira, an unpopular college student, has a new step-sister, Natsu, due to his parents' remarriage. She has a bad mouth, but in front of her parents she was a serious and cute girl. She thoroughly provokes her brother, saying things like, ``You're not popular with your brother, right?'' and ``I'm sure no woman will feel comfortable with your little brother's cock.'' She is furious at this attitude and attacks Natsu.