Longish black hair, purplish-blue eyes, slight and feminine build. Chihiro wears a white and light green hooded jacket hiding the majority of her slight and feminine body. She wears fitted light blue jeans that shows the perfectness of her butt(or at least according to Setsuna Ena).
She is shown to be very friendly, coming over to clean up and cook for Itsuki. She likes doing many feminine things such as cleanings and wants Itsuki to make up with his father.
Basically the little sister that Itsuki has, yet to come out as his little sister.
Itsuki is unaware of the fact that Chihiro is a female because when she first met Itsuki, when her mother (Itsuki's dad's new wife) and her were presented to him, he suddenly left them and refused to talk with the whole family, grudging against his father for replacing his deceased mother, until he finally left home. Chihiro is only in a good relationship with his step brother because she went and visit him regularly when he transferred to a new home.
There are many parts when Chihiro seems uneased at not being a girl. It's obvious when Ashley Oono called Itsuki Hashima as "onii-chan" and when they played TRPG as Chihiro chooses to be a girl to be Itsuki's little sister.
Itsuki Hashima - Chihiro's step brother tho whom she may have a crush on. (Speculation)