His father has remarried and started living with his new partner. 
Her stepmother has a daughter who turns out to be a demon teacher, it is very difficult to live with her, Reina-sensei
But outside of school, Reina was totally different she was very kind, defenseless and afraid of insects.

A young boy is initially overjoyed at the news of him having an older sister, due to his father remarrying to a woman who has a daughter. That is, until he finds out that his new sister is actually his school's strict PE teacher, Shina. With his father going off to live with his new wife, the boy will live with alone with his new sister. In their new life together, the boy finds out that Shina at home is completely different from Shina at work: clumsy, scared of insects, and kind.

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