
I think that actually is Seo-in. You can see his eye mole and because of the way they're standing their moles by the mouth can't be seen from the angle (left side mole of seo-in — right side mole of jaewon).
And judging from how Seo-in is grabbing the shirt of Jaewon it may be Seo-in who initiated it. But now with the context of their backstory, I doubt Seo-in did it out of his own free will

The thing is that kissing scene is shown multiple times throughout the chapters and the mole is misplaced.
Chapter one has the mole on the right side (which seoin does not have)
Chapter TWO has no mole added at all in the kiss scene. Then art right beside it is the mole added on the left. However I genuinely don’t think that was part of the kissing scene. Because seoin has no clue why MC left, making it not make sense that he would’ve smiled and saw MC after a kiss with the brother. I think that it was either a separate scene from the kissing one or MC imagining him smiling with the brother.
And Chapter 20 has the mole on the right side in the scene again. So I guess it’s more confirmation that the mole in that kissing scene should be on the right side since two chapters had it that way. I’m guessing the artist forgot to put the mole for chapter 2.
(tell me if I'm not mistaken) but in the ep 2 flashback- was it Jaewon who kissed his OWN Step brother (Seo-in) to piss Yeongwoon off? Istg- He's one sick mf..