Kusakabe Rio

The youngest of the 3 sisters. She is part of the housekeeping club. She doesn't talk much with people of the opposite sex other than her brother.
Since her elder brother is unreliable, she takes care of him. Instead of having romantic feelings for him like the rest of the sisters, she is more interested in keeping the welfare of the family.

Spoiler

When a Teacher finds about Satoru and Ami incestuous relationship, she is blackmailed into having sex with him if she does not want it to become a scandal.