Sangwoo is autistic. As someone who's dealing with the same thing I should tell you that for us some stuff are a big deal that others really don't care about. As an example in my workplace I'm in charge of organizing the shelves because "nobody can do it better than you do" as my boss says, and yet he absolutely understands why I get mad when someone changes even one small thing because he has seen me having an anxiety attack over something that is not a big deal to him or anyone else in our workplace.
I'm a little surprised that you say you read the whole thing and still say that Sangwoo did absolutely nothing for their relationship: Sangwoo changed his belief about love, he became friends with someone close to Jaeyoung, emotionally supported him when he was at the funeral and tried to keep up with Jaeyoung's expectations of a relationship. They may be nothing to you but they're still a big challenge for someone who believes that a relationship is only between a man and a woman.
Everyone has their own preferences about texting, the only time I personally didn't give a crap about how someone is texting me was when I either didn't have any feelings for that person or I absolutely hated them, otherwise I have and will be telling the other person that the way they're texting me is bothering me because if I don't every time I get a text from them will make me upset or even mad and at some point I will ask them to never text me again. When we love someone everything about that person becomes important to us, it might make us annoyed or sad or madvor excited or happy, and none of them means that we don't care about the other person.
To me it seems like you have a difficult time understanding Sangwoo and that's totally fine since, as I mentioned, he's autistic and you probably have never came across someone like him, but calling him names and saying that many bad words just because you didn't notice just how much he did for hos partner and their relationship or just because you have decided that you hatr him for whatever reason are a little too mean.
Seems like you love his partner though which also explains why you completely ignored the bad things he did in their relationship.
None of the things Sangwoo did and is doing makes him an a-hole and nome of the things Jaeyoung did or is doing made me hate him, maybe trying to understand them would help a bit.
I also have just decided to accept that yeah he's autistic. I hate the narrative decision however to write in the fact that he apparently does not however. That decision by the author is what frustrates me more than anything, because I am now being told that he is neurotypical, and so the decisions and character on display should be viewed through that lens. If it was left either confirmed as neurodivergent or just ambiguous, then I would be more understanding because I would read him as autistic. The author however explicitly states that there is no condition to explain his actions, hence why I did not approach him as a neurodivergent character, despite clearly showing signs of that. Why write him as explicitly not neurodivergent and then expect readers to read him differently? You may be autistic, but the author explicitly states that he doesn't have a diagnosis despite going through testing. I therefore am left to read him as intended, that this is just his fundamental personality where he is inconsiderate and selfish imo. And making him neurotypical tells me that his stubborn ass is preventing his growth. I'm not saying that he didn't change absolutely at all, but the rate of his growth is so beyond small that the relationship just does not feel balanced at all to me. And he doesn't even seem to be aware of just how much compromise Jae has made for him this far. I don't agree with the writing decisions where it concerns his diagnosis. They really should've left it ambiguous or explicitly stated that he was autistic, instead they wrote him as not being so. Seems unfair to ask readers to read him as autistic when the author goes out their way to say that he's not.
It also makes your comment seem like projection because the author writes it into the story that Sangwoo isn't autistic. And yet here you are just stating that he is because it makes the most sense to understand his behaviours thus far without faulting him. But the reality is that no matter how much you head canon him as autistic, the author wrote that there's nothing, just pure personality traits. Sorry but I read it based on what the author wrote, not on what you think it should be. The story went out of their way to state that he's not. Which was honestly a poor writing decision.
Actually meant to write stubbornness not stubborn ass but whatever. And I said that Jae's not perfect already, I just didn't wanna address in much detail because I was focused on Sangwoo and some of the writing decisions for his character and the impact of said decisions on the relationship and my overall reading experience. This frustrated me because I really was growing to love them, and then the author would just reverse development or throw in some communication issue and it's just annoying when it's done over and over. I don't hate Sangwoo, I hate the way that he is written.
I read this in one sitting and really loved some parts. I really enjoyed Jaeyoung's character, got myself to mostly tolerate Sangwoo...but this side story is just not it. Like obviously a relationship will have compromises, but it should be balanced. Their relationship is definitely not fair and I feel like the side chapters are reversing Sangwoo's already piss poor, mediocre, miniscule, shitty ass shred of development. There is such a thing as picking your battles and I'm sorry but he needs to understand that he's not always right, and it can't just be his way or the highway. Jae is always having to do soooo much more for him. And their fights are really stemming from a lot of little things that he just seems so unwilling to even recognize. Like cmon, spacing? That's not a hill to die on. Why rile up your partner that you claim to love over such a small personal difference? He's passive aggressive, selfish, immature, uncompromising, inconsiderate and narrow minded. This webtoon is carried by Jae, just like their relationship, just like Sangwoo. But a relationship should never be that heavily skewed. The mental and emotional work can't be driven by only one party. Sangwoo needs to learn the art of introspection and being a human. And this isn't even to say that Jae is perfect, but his flaws pale in comparison to Sangwoo's. At least he can recognize when he's wrong or overdoing something and can apologize for his reactions or amend his future behaviour. Sangwoo though? Like a fucking toddler. So emotionally stunted. I find it a little unbelievable that he wasn't diagnosed with some type of behavioral/mental/social disorder of any kind. That writing decision is probably what lowers my willingness to accommodate him too. Because the story literally has a way of sometimes validating his very flawed actions as somehow acceptable and even makes him a victim when Jae should definitely have been priority in the emotional exchange. Some of the writing decisions for his character are just so weird.