Takatsu Narumi

The protagonist's former step-sister, who became family when their parents remarried, and is now a stranger since the divorce.
She still cares about her protagonist like a brother.
On the surface, she is a serious and dependable sister, but to her brother, she is a lot more playful.
Narumi possesses an unusual affection for the main character, and she even offers her virginity to him when he was an adolescent interested in sex.
This leads to her repeating various sexual acts with the protagonist, and awaken her perverse side.

She'll do anything for her brother, so she's a convenient pawn since she can move at will throughout the school.
The protagonist treats her like a slave, but she is happy to be treated that way.
Also, she herself is greedy for pleasure, and begs to be screwed hard.
She is the adviser to the drama club and lends the club room and costume for the protagonist.