Some ideas if anyone is interested :D

Since this end to ch 13 did kind of unsettle me, i started at ch 0 again. (excuse typos bc english is not my first language and i am on my phone)

1. Guwon is something like a half-zombie.
He crawled out of some rubble and might have already died, but he has some sort of immunity to the brainless part of being a zombie. Still he lost a majority of his memories and is clueless of some things (could as well be an act or he simply is not the brightest with general knowledge, but with other things, who knows).
Though I guess he could be aware of being a half zombie, since we never actually have seen him eat anything.
If you go by other zombie stories, the zombies mostly have to throw up after ingesting human food, so there is that possibility.

So when smooching Jinwoo, ingesting his saliva made him "hungry", so to keep control of himself he had to puke up the spit or something, and obvs he is proud that he managed to have his man and keep his shit together.

He mentions in ch 5 that he needs to keep a clear head at all times (this was mentioned in regards of drinking alcohol).

2. Guwon is not human anymore to begin with.
Maybe he simply died and woke up like any other zombie, but had regrets of his past life that he still wanted to fulfill.
I mean, we see him in the beginning walk to that destroyed house and he says that he always wanted to live in a house like that. So maybe not just a house but a town where he is liked and respected. (with his built, maybe he was like a gangster or something, that wanted a peaceful nice life after all.)
And that brings me also back to the point above about smooching Jinwoo and having to get his shit back together by throwing up.

3. Guwon got his "humanity" back.
In ch 6 that girl in the black hoodie says that the Zombies would be killed if you cut their head off, guess who has a scar on his neck that looks like his head was sown back on. Some mad scientist somewhere that tried it out and it worked, but who knows.


4. This one is a bit abstract, bear with me:
- Guwon is a halfie that tries to make Jinwoo one too.
Jinwoo was on the brink of death and also badly hurt. I don't think it is established how much time is between Guwon picking him up and them being on the car or at the town, but his head wound healed quite quickly.

I know he probs has a concusion and amnesia from it, but he says himself that he forgot some stuff, which might be the zombie virus eating parts of his brain away.
There was also the instance where he slept for 2 days straight (also reasonable with sleeping off the trauma and just catching up on sleep) but woke up to wanting to stab Guwon.
Jinwoo sleeps a lot in one go (I can only dream of that but that's not important), so maybe his body goes through some sort of process during sleeping that fixes him up. His leg also healed basically overnight.

Also, do we know what Guwon might have done to Jinwoo in the time between saving him and the next scene we saw?? Maybe saliva/bodily fluids is one way to make a human also a zombie (or halfie, whatever). So maybe Guwon was aware of the possibility and smooched Jinwoo to heal/safe him.

I mean, he could just kill him if things would have gone wrong.

While I am at it, the whole town could be made up of halfies (lack of memories, also excuseable with simply just forgetting the trauma) and the rule about leaving the door unlocked is so the rest can ensure that if one becomes an issue, they could get rid of that person.

Anyways, I think, Guwon truly likes Jinwoo (all that flirting and putting a ring on him, indicates that to me at least) and there was a good reason for whatever tf he had to throw up for.