Kagurazaka Yui

An student-teacher assigned to the protagonist's class.
She is a young lady who grew up in a box and has no knowledge of the world. She has little self-assertion.
She has had a complex since childhood about her breasts, which are too large for her body.

She has no experience in sex, and her knowledge of sex is limited to the level of health education.
She has a fiance, but the relationship is platonic, with no choice other than her parents' decision.
She is not dissatisfied with her fiance and accepts it as such.