Shimazu Aki

Hidetaka's younger sister.
She is afflicted with an incurable illness. For some reason, Hidetaka and Aki seem to despise each other; nevertheless, they maintain regular daily interaction as needed.

She is a student at Keiou Academy, an all-girls school where Hidetaka worked as a temporary teacher.

Aki's mother died when she was a child, and when her father married Hidetaka's sister, she and Hidetaka became siblings. Her father and Hidetaka's sister were later killed in a building explosion. Aki attempted suicide once, feeling guilty since she believed she was the cause of the incident. To prevent her from committing suicide, Hidetaka physically abused Aki for a while, making her hate him to the point of wanting to kill him. However, after realizing that Hidetaka did it protect her and he never meant to hurt her, she changed her attitude and became the polar opposite of what she had been.