Juujou Shion♀

Shion is one of Mizuho's classmates who figures out that he is a boy almost immediately, after their second meeting, but tells him that she will keep it a secret from anyone else. She tends to have a silent demeanor and voice which helps to give her an appealing appearance to the rest of the students. The previous year, she actually had to leave school for a time soon after she was elected as the Elder due to an illness and had to spend her time in the hospital. In effect, she has come back this year in order to repeat her last year, though she still isn't fully recovered, as she will pass out if she exerts herself too much. She becomes an adviser and a female role-model for Mizuho.

She has been engaged to Kazutaka Itsukushima who is Takako's older brother in Shion's path; Itsukushima family is upstart, and so Takako's father made Kazutaka to be engaged to Shion in order to get Juujou family status because the Juujou family is a former aristocratic (Kōshaku (侯爵)) family. Takako asks Mizuho to rescue Shion from the Itsukushima family.

[Taken from Wikipedia.]