Sora

There is so much to their relationship, which is currently a love-hate one. Like you can tell by the way they touch each other, especially so casually and/or gently. They have familiarity already, which is expected after 4 years of being together nonstop, and that familiarity is about to turn into sexual relationship, which, of course, will start them on that path to lovers. Now throw this potion in the mix and there will be angst and betrayal, while also trying to deal with Yehwa's flashbacks and Muyeon still wanting him back. They're fucked up love story, if done right, could be absolutely insane!

Okay, theory from someone who has never read the novel, based of the plot and hints they're giving here:
Youngwon being unhinged absolutely makes perfect sense and It's hard to not feel bad for him tbh. He is stuck in this zombie world, gets to live out however many days at a time until Hohyun dies, and then starts over again, probably meeting him at the top of those stairs. He himself has probably been eaten by zombies before/killed by other people (ie the scar around his neck) to the point he doesn't even care anymore. He doesn't remember shit about himself, but remembers everything from the replays, which means he's probably been doing that for an incredibly long time. Hohyun is stated to be and depicted as a careless character in this survival situation, and has probably died in ways that could have been avoided, which would make Youngwon start over again. Youngwon says he never met him before the zombie day, so all the feelings he developed for Hohyun were most likely during these replays, so in reality he probably both really loves and hates him simultaneously.