Is Ritsuka-kun a boy or girl?

It's obvious when it stands -kun behind the name. Ritsuka-kun is a boy.

Well, I'm going to change this answer. Just because the name is Ritsuka-kun doesn't mean it's a boy. It goes the same way for "chan," because it is used for little girls/boys aka Tadashi-chan can be a boy even though "chan" is in the name. the suffix "kun" for girls can be used when a male coworker is addressing a younger female coworker, or when a girl is a tomboy. (ex: Akira-kun) I hoped this helped. :)