This may sound like a stupid question, but here goes - what does referring to someone by surname without honorifics usually mean? About the speaker's personality or relation to the addressee. Because HaruMaki, Hoshi, and I think Hinata refer to people like that, so I'm curious as to what it can mean or represent
Yobisute is generally considered pretty rude to use with people you don’t know, and perfectly accepted between people who are close–this is because it’s kind of like an expression of familiarity/being equal to or above the person you’re speaking to.
In real life, honorifics and how they’re used is pretty complicated and messy–you’d never go up to your boss or teacher or whatever and just call them by their surname unless you had a particularly pally relationship with them, but on the whole not everyone follows the “rules” as it were.
In fiction, meanwhile, usually these decisions are made in order to tell you something about the character.
Hinata probably doesn’t use honorifics because symbolically he’s trying to very hard to convince himself that he’s on everyone else’s level. HaruMaki doesn’t use them because she is 1000% done with these losers and ain’t here to make friends. Hoshi doesn’t use them because he’s done hard time and is outside the normal mores of society. Momota doesn’t use them because he’s just the type of dude who speaks really informally/rudely. Yumeno doesn’t use them because it’s probably too much of a pain in the ass to bother.