One of Shougo's friends and a tsundere. She is in love with Shougo and even said she wants to marry him. Her family owns a clothing line.

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.
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.
In the light novel, Miyabi became Shougo's step-sister after Kanako (Shougo's mother) married Miyabi's father.