An 11-year-old sixth grader, Rio is the tallest girl in Natsuru's class. She secretly lives alone with her little brother, Yuuta, in a rundown house. She is amazingly responsible and has gone through unbelievable things. She let Natsuru stay over at her house when he decided not to attend his summer camp and later went to the Bon festival with him.

Spoiler

It is later revealed that she buried her grandfather's remains in their backyard to avoid generating rumors against her father.