
Good girl? I mean she's indeed in love with Mincheol but she's not a good girl. Even after knowing that Haesoo was his ex-wife she didn't give a damn about her and kept pursuing Mincheol.
She's so stupid thinking that a guy that cheated on his wife would be loyal and good to her.
Mincheol still loves Haesoo in his twisted way, he doesn't feel anything for that other girl. Sure Mincheol is a bastard and he doesn't deserve happiness right now but even that other girl doesn't deserve happiness, she's getting what she deserves right now. Karma is a bitch we all know that.

You're right maybe he did, I honestly don’t remember right now, but I do clearly remember that Mincheol and Haesoo had a very obvious fight in front of Ari, where Haesoo said something like, “How could you do this?” What I’m trying to say is, it was obvious they were in a relationship, especially since Ari was literally standing right there and heard everything.
I understand that Mincheol might have manipulated her a bit by claiming he and Haesoo weren’t together, but she’s still a grown woman. She should’ve paid closer attention to the situation. She saw how hurt and angry Haesoo was when she saw them together, so deep down, I don’t think Ari actually believed Mincheol’s explanation. She just chose to accept it because, selfishly, she still wanted to be with him.
Honestly, the second girl seems to really love mincheol. He is gonna lose another good girl at this rate.