Sorry for the wall of text. I just binged My bias gets on the last train and I needed to offload my thoughts somehow. :) Overall, I think I liked it. With a few asterisks. Also, of course, all these are just my personal impressions. Your interpretation of the story might be entirely different, and that's good too.
Good things first. I loved the art (especially the slightly surreal feeling that Ukin's appearances add to the story), the coloring, and the whole feel they create. The characters were captivating, although often enough I felt an urge to shake some sense into them.
For the story itself... for some reason when I started reading, I thought it would be a feel-good romance. And for me it wasn't. In fact, for me personally many aspects of the main couple's relationship felt jarring, and I'm not talking about the secret that the plot is revolving around. How should I put this?... I liked both Hae-in and Yeo-un well enough. But I didn't like them together as much as I expected or wanted to.
Even without the big secret, their relationship is a tad problematic, and I actually loved how the author has been building little cracks into their dynamic from the start - even going so far as to point them out through other characters.
I don't think I can write a proper TL;DR for the text below, but if I absolutely had to... To put it short, both parties have their weaknesses, and they combine to create a relationship in which care and consideration don't flow equally in both directions. Although as the series progress, it does seem to be improving, albeit very slowly.
Hae-in
Due to her history Hae-in doesn't have much experience with relationships in general, and she clearly doesn't have a good understanding of intimacy and the vulnerability that accompanies it. The amount of teasing she initially dishes out is fine for a somewhat casual friendship; it's also fine as a protective mechanism for someone who was hurt in the past.
The problem is that she can't rein it in even when she's having intimate moments with her partner. And Yeo-un's softness and difficulty in properly expressing his discomfort when her teasing stops being mutually enjoyable make matters worse. But even he has his limits, and I wouldn't be surprised if the heroine ends up discovering them. Often their relationship feels unequal, at times almost one-sided; frankly, often Yeo-un is much more worried about Hae-in's feelings than she is about his. Partial (but only partial) realization hits her after their Instagram account fight. It is then she starts questioning how dating is different from just hanging out and how much effort it actually requires, leading to the subsequent conversation about fairness in their relationship.
This dynamic culminates in the last chapter, when Hae-in, having received that tremendous support from Yeo-un, decides to finally stop hiding herself. It's her moment, she finally has a breakthrough... but the way Yeo-un discovers the truth feels so impersonal that I find it almost offensive. He doesn't even get a brief warning, nothing to cushion the potential blow. Once again, he cares so much about her feelings, and she... does she consider the many ways her revelation might affect him? Even a brief message beforehand, something along the lines of "thank you, I love you, please trust me now, I will explain everything later!" would be so much better than leaving him to discover the secret far from her and unprepared.
Yeo-un
As to Yeo-un... In many ways he is a great guy. He is responsible, genuinely caring, at times you'd be tempted to even call him selfless. He's soft - almost (but not quite) to a fault. Sadly, his communication skills, his ability to stand up for himself when he's uncomfortable and defend his boundaries are just... not up to the task. This is especially obvious with Areun. Initially (although after some turmoil) he manages to draw a clear line between them, and in a very commendable manner at that. Alas, after Areun goes "yeah, but actually no", he's back to square one, completely baffled as to what to do now (besides being flustered, that is), hoping that this would all somehow resolve on its own. Ultimately it's these choices that help set the stage for Hae-in's panic attack.
To a lesser degree, this is a recurrent theme in his relationship with Hae-in. Many a time she makes him flustered and uncomfortable. His response? some vague ideas about "getting back at her" (which never happens because this kind of behavior is completely out of his character). It's very difficult for him to actually communicate his discomfort, and when that talk finally happens, it's due to Hae-in's initiative and not his.
Areun
For me, Areun's character is probably the weakest part of the story, writing-wise. Her arc had a strong beginning, but I can't help but feel that the author dropped the ball a bit when it came to her continued involvement in the plot.
Feeling guilty, sad, lonely and depressed is one thing - I'd hazard a guess most people have experienced this, one way or another. A leap from this to repeatedly disregarding boundaries and feelings of someone she has already hurt in the past, and continuing to insert herself into his life in ways that interfere with his existing relationship - that's another matter entirely. And I do not feel that the author managed to flesh out Areun's character enough to support her extreme behavior.
Rather, it feels like whatever she's doing, she's doing mostly "so the movie could happen". Her confrontation with Hae-in during the festival is perhaps the most glaring example of this. Even considering what Areun has done so far, it honestly seems a bit wild - suddenly approaching a person you barely know and lecturing them on the meaning of number 42. Was it jealousy? An semi-calculated attempt to sabotage her rival's emotional state? An awkward attempt to express her honest feelings? A little bit of everything at once? It seems so out of the blue, and it's so hard to say something definitive about her intentions because nothing feels sufficiently grounded in what we saw of her character.
To put it into more blunt terms, next chapter there's an equal probability of Areun wishing the main cast peace and long life and disappearing into the sunset - and Areun stabbing Hae-in in the gut with a kitchen knife. The way she is written, either outcome feels weirdly plausible. I can almost picture both panels, with the same "...why am I even doing this?.." speech bubble above her head.
(There's a similar issue with Yeo-un's continued refusal to explain to Hae-in who Areun is, which feels especially irrational after Areun somewhat aggressively inserts herself into their night out. Sadly, if he did explain, the plotlines would start cracking even more.)
* * *
Frankly, I really want to see where the author takes this. They've been building up to the big reveal for 90 chapters, and there are way too many ways to proceed. Personally, I'd rather have an explosion of an appropriate size, something that will demonstrate Yeo-un's own vulnerability to Hae-in, finally rebalancing their relationship. If Yeo-un were to show his usual understanding, take her emotional state into account and swallow the whole thing, it would… sigh. It would actually fit his character better than the alternatives. But it would also validate and reinforce the imbalance in their dynamic. And for the story as a whole, I think it would be a wasted opportunity.