Uh, bad news - I didn't know either but this is a boy. Apparently he's that guy's little cousin who is starting to look more and more like a cute girl. Which is the joke here. So yeah, the bubble belongs to the guy looking at cute 2D girls on his phone to reassure himself he's not into boys.

Don't ask me, never read this. It's just a random reaction image. And reverse image search says it's from Inaka ni Kaeru to Yakeni Jiben ni Natsuita Kasshoku Ponytail Shota ga Iru