One of Shougo's friends and a tsundere. She is in love with Shougo and even said she wants to marry him.
Spoiler, click to view
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 view
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.
Spoiler, click to view
In the anime, it is revealed that she is Shougo's real sister.