Yes, she is, and Ryo is 18.
(I'm kind of fond of the ship, in a weird way. They have a good vibe in their canon interactions, and Koume's ability to inflict terror smooths out a little of the power imbalance. My headcanon of a relationship would be Koume with a crush she isn't sure what to do with, and Ryo mostly trying to be a friend but also not knowing what to do.)