That's... Takayuki. His best friend throughout this whole series, who just started dating Shimizu. Did you already forget him?
Edit: Oh, you meant right side not the other side. I have no idea
They got the same puffed up hamster face... also I mean... obvious plot point as to why she's so yandere innocently interested about him. It's the classic long lost childhood friend comes back and is still in love with MC story, but with a collection of unrivaled reaction faces.