@Tcof : They're roughly three years apart. Chion finished high school at the same time that Gen finished middle school. She then stayed at the temple helping Grandma instead of going to university. It's mentioned early on in the manga, so her being twenty next year fits well with the story.
@cixous: He's into Hiruma's friend. He even told Gen and the other guy a few chapters ago. He thought she was next to him and was trying to make a move.