As much as I love the writing though I can never seem to really like the side characters that serve to complicate the relationships into complex triangles (Dawoon, Nami and Sunhwa) because all of them also seem to be written into the plot without past evidence and just more as plot device (such as in the Case of both Dawoon for Sungji and Sunhwa for Sumin) or written out of it (such as Nami not even seemingly being 'the closest' and 'the only friend' Sumin had in highschool as she and the other characters always claim because she didn't fit the narrative properly, I'm assuming, even just a silent one panel scene of her talking with Sumin in the hall could have helped bridge that discrepancy I think)
I forgot three other important firsts, which would have been her even being held as warmly as Sumin had during that first anniversary, she seemed so genuinely puzzled at what Sumin had wanted and seemed so pleasantly surprised, the first time anyone had held her in such high regard, as well as the first time she'd ever even hoped and looked forward to a future, which I still find completely heartbreaking.
I think the side story did a wonderful job at giving us a better understanding of why Seju couldn't move on for so long and fought so hard (I think it was always already established that Sumin meant a lot of firsts to Seju not just romantically but seeing it first hand really drives the point home) How Sumin must have been the first person she considered a friend, the first person she genuinely liked spending time with, the first person to tell her not to go, the first to say they liked her, the first to actually make her happy, the first to actually defend her, the first who made her their priority (it kind of colours her in a way where in, even broken and dragged through the mud in the years that followed, their relationship became something difficult to let go of, for someone like her who's been so scarcely loved her whole life) but also, I like how it also gives us the reasons why Sumin was so heart scarred as she was, how well her relationship with her parents were, how much she valued Seju, how much (forgive me for not using a kinder word) sex meant to her (how it's something she had to mentally prepare for for a year and did so nervously) and how much devastating Seju (accidental as it was) cheating on her was (as if sex didn't matter and was that easy to give), it even shows during the time she was dating the woman with red hair (I apologize but I care not to search for her name) when she was seemingly coerced to use a toy and she suddenly imagines Seju in the woman's place, doing it with a man, before breaking out in a cold sweat and feeling sick right after, Seju was love starved who had understood happiness for the first time and Sumin was soft and doting and it makes everything feel so devastating because if they only had some time, if Seju's 'brother' didn't interfere the way he did, they might have been able to find their way back because once upon a time Sumin really did intend to end things properly and during that time she wasn't as broken as she was after her assault, and Seju wasn't as twisted as she was after Sumin came back the first time. But more than anything I like how this truly gives us a look into how far they've come, how much was at stake at truly ending things and how everything they've gone through to get here changed them into people who could finally let go.