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One day, a picture book writer named Yorozu Hiraku was asked by his close online friend, Chintarou, if they could share a room while he attended a nearby university. Although they only knew each other through online interactions, Hikaru agrees to it

A 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 strict PE teacher, Shina. With his father going