I have been thinking the same thing about the psychological tag! Like how can he mask his presence around people? He has to have some sort of gift or is of another world.
I think im going into tinfoil territory but for sure the black haired guy is not human but supernatural. Its obvious that he can manipulate how he looks like to others and Hanjoo is the only one that can see his true face. Hanjoo is seen as dark and creepy by others except Sunbae which is why they became friends. In the raws, sunbae is the only one who sees him as bright and handsome, literally everyone else feels something off about Hanjoo. Sunbae thinks Hanjoo should be popular bc of his looks but no one else thinks that. I think the black haired guy masked Hanjoo’s image (similar to himself) when he was young to prevent him from being close with others but only sunbae is immune to it. Just a theory.
The more I read the more I'm convinced the older brother is some creepy entity without human emotion that's gonna go full psychological horror mode on the MC and his lover.
It's been slowly building with him getting more and more creepy, and I'm pretty sure he just tried to kill the seme by summoning that puddle on the stairs lol.
MY guess is it'll backfire and only bring the two closer. But yeah, I'm sensing some psychological horror coming ヾ(☆▽☆)