One of Shougo's friends and a tsundere. She is in love with Shougo and even said she wants to marry him.
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It's later revealed that she met Shougo when he was recovering from an injury in the hospital and he played with her; they even had a pretend wedding. Spoiler, click to viewclick to hide
According to the Vol. 8 of the novel series, it is revealed by DNA test that she's not actually Shougo's sister - but his cousin. Her family owns a clothing line.
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In the anime, it is revealed that she is Shougo's real sister.