Sonoda Reika

She teaches mathematics at Kawachi Juku, a preparatory school. She is the younger sister of Sakurako Onami.

In contrast to her older sister, who has a gentle image, she is a cool woman who does not show her emotions, but she has a troublesome personality that sometimes plays with men's feelings with her charming attitude.

Spoiler

She is the real mother of the protagonist. Angry after losing her lover to her sister she got revenge by getting pregnant. She then forced her sister to raise him.