Just like how Seju was in the main story of WDTFS, Yeonwoo sunbae is clearly not the villain here. She's just a broken girl who lost her GF whose clearly dear to her heart and is coping the pain by sleeping with Ji-ho. She needs healing and that person who can give it to her surely isn't Ji-ho.
Like for real? What has she done that warrants such an immense dislike towards her? Judging by some of the comments on here, you'd think she committed murder or something...
As far as I'm aware, she offered Ji-ho a physical relationship since she didn't want to commit so soon having lost her GF recently. And Ji-ho accepted it, knowing they won't become an item but welcomes the opportunity to fulfil her sexual needs.
She's also not intentionally getting in the way between the main couple and purposely sabotaging their relationship because firstly, Ji-ho didn't exactly tell her she has a crush on Woohui all along. Woohui herself is acting confused if she sees Ji-ho as more than a friend and continues to date guys to give the impression she's still straight.
Besides, one way or the other, the main couple will get together in the end. Why not just leave the poor pink-haired girl (Yeonwoo) be and let her enjoy whatever brief moments of happiness she has with her junior before their fun ends?