Eunjae was just waaay to much of a beta-male for my tastes. I wanted him to make at least one real effort for Minseo. I feel like him ending up with the heroine (a.k.a. Hayoung) ultimately vindicated his "let's just give up without trying" again attitude, which is something I cannot agree with. If he was to end up with Hayoung, it should have been after he realized what a fool he was (which he did not do even at the end), and really tried to win back Minseo, not just with a half-assed confession.
I think Jieun Song's character was massively underdeveloped, and all in all paper thin. She didn't have any deeper motivations, she was just kind of evil, because her mom died. It's a bloody shame, I'd loved to read her and Hayoung slowly becoming friends instead of enemies, but she didn't even get a redemption arc, she just kind of stopped being evil after Jaehyun left her for good.
Speaking of Jaehyun, he would've been a way better choise for Hayoung. They were basically star-crossed lovers (with Hayoung's supernatural dreams from an alternate reality), and the only reason they don't end up together is a Hayoung's deliberate self-sabotage.
All in all the ending is not horrible, but it's really not my taste. None of the "first loves" work out, but not because they are impossible, just due to self-sabotage, and just plain old sabotage, which to me really doesn't say that it "wasn't meant to happen", but rather that these guys are really really stupid.