He seems to be designed to be unlikable, honestly. She's making a joke and keeping things casual when that dude invites her somewhere, an important skill for a young popular girl, but he decides she's a fucking bitch for it for some reason. He then assumes that the girl who he knew since they were kids was going to believe he'd steal her shit and is surprised when she's like "what? No, I know this guy! He's a good guy! How's it goin' man" and jacks off to the idea of using her cosplay account as blackmail and punishing her for being a bad girl. He is ashamed of doing so and fantasies are just fantasies, but its a strange display of like... ownership.
He could just ask her, send her a message on the account, but is too afraid to. He's immature and insecure, and rather than trying to deal with those, he gets angry at her and disappointed that she's not his ideal of a cute girl like when she was a kid. Even though he has no idea what she's like cause he hasn't talked to her in fucking years now.
I think you're supposed to be hoping he gets his shit together, cause he's not in a great personal spot.
