Suzuhara Miko

The younger sister of Reiji's friend Suzuhara Heiji. She's the same age as Riina leaving her with 2 years left to complete elementary school. However the two are not in a class together. She's just starting sex education in class and although she's still too young to get pregnant she wants to practice baby making with Reiji. It helps that he's a lolicon and she knows it.

Her 'assigned number' is 11.

Spoiler

She enjoys sex around the house when Riina is around because she hopes to be caught so she can share Reiji with his sister. She enjoys watching the siblings have sex because the immorality excites her.