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Because of their parent's remarriage, high school student Satou Yuka, suddenly finds herself living together with her stepbrother, 23-year-old high school teacher Satou Taichi. Actually, their parents have not registered their marrriage and therefore they aren't even siblings by law; but since both parents aren't around, they end up living together! Taichi who has a very serious case of sister complex, wishes for Yuka to call him "onii-chan." But Yuka's reaction is very cold?