I'm not quite sure how to interpret the "rejection"
I think she’s just gay and oblivious. Maybe we need that nameless wlw couple from Green Tea Bitch to show up and teach her that those feelings are romantic
Nah, she knows. I think she's into Jeongwon, and is aware of it, but she's actively trying not to be. Remember Hwayeon had a flashback about kissing Jisoo while in tears. They also have each other's numbers and texted each other, so there's history there. Jisoo also admitted when she was drunk that she had somebody she was trying not to crush on. Jeongwon assumed it was Jeongwoo, but that relationship is definitely platonic, almost familial.
My working theory is that Jisoo broke Hwayeon's heart. (And possibly also her nose, most likely by accident, I can't see her striking in anger or malice, but the way everyone talked about the incident sounded like there were witnesses to Hwayeon's bloody nose, so I don't think it's totally made up.) This might be because of Jisoo being hung up on her dead friend, or it might be a result of Hwayeon's issues, or both, or something else, but in any case there were definitely tears and blood.
Then Hwayeon gets into her current thing with Jeongwon, and from Jisoo's perspective, it seems like they're probably dating in secret. But there's all the stuff about the resemblance, and Jisoo can't keep her eyes off, and eventually the universe throws them together by coincidence, and she figures maybe they can be friends. But she isn't looking to wreck Jeongwon's relationship, so she's trying to keep it platonic, even though she is -definitely- crushing.