
Looked up Sisyphus and there's already so many references and potential foreshadowing in just the first 5 chapters.
He's the founder/king of Corinth and the one in Greek mythology who is punished to roll a boulder up a hill, only to have to fall back to the bottom, for all eternity. (leader of the settlement, lives at the top of the hill)
He angers Zeus and is sent to Tartarus (prison in underworld), where he outwits the god of death and chains him up instead. While Thanatos is chained, no one dies on earth. Eventually Ares frees him and death resumes. (people suddenly stopped dying, ML has something to do with it?)
When he eventually dies of old age, he schemes his way out of the underworld by appealing to Persephone, who allows him to return to Earth. (scars on neck, has already died?)
Only reference I could find of the 'Hounds' was from Anne Rice's The Passion of Cleopatra, which was incidentally also about the dead coming back to life.

I feel like a lot of these types of plots end up with it being like, "oh no, you totally misunderstood! It's really YOU that I've loved all this time!" xoxo happily ever after.... but I REAAAALLLYY don't want this to end that way... there's just such clear pining for the older brother that it would feel like a cop out

Yeah, I hope the misunderstanding clears up, but we still get the friend FINALLY puts His feeling for the older brother and slowly (or at least with a Time skip idk) develops feelings for the younger brother. And I'm not saying he should suffer for it, but I do wish he feels a little bad about constantly talking about him, as a younger sibling myself there's nothing more annoying than people just coming to talk to you because of your sibling so I feel petty as fuck.
Maybe he's possessed? Like Guwon is the name of the body and the demon or whatever is learning about humans through him? So the demon is like, "damn son, your body/face is really coming in handy!"