They will have an argument now, when Jenny's secondary personality is triggered, and she will say very mean things to Inseop. Inseop will answer (not rly rude but in the future he still feels guilty about it). In any case, later Jenny's real personality comes back and she's apologetic about what she can remember it happened, repeatedly comig to Inseop's house to gain his forgiveness, but he already decided to not speak with her again (this too triggers a lot of back and forth between Jenny's two personalities and she sometime apologises, sometime blames him). Later something goes wrong in Inseop's surgery but he is barely saved at the last minute, and when he wakes up the first thing he hears is his mother's convo where Jenny's death is mentioned. He later believes firmly that it was because of Jenny that he was able to survive (like she gave half of her soul to him). Then at the funeral he meets Jenny's aunt and realizes that Jenny wasn't as crazy as he thought he was (he believed Jenny made her up and told her that too during their fight) and feels even guiltier about what he told her. Her aunt gives him Jenny's diary since she saw it's a lot about him, and there he sees a clash of her two personalities - the real one who loves him a lot and tries anything to bring their friendship back, and the second one who swears at everyone, blames Philip for everything, and sometimes is mad at Inseop too. Inseop decides to no longer ignore Jenny's words like he did, and starts the revenge plan (honestly, the novel implies that he did it more as an emotional support for himself, and it's clear that he still has feelings for Philip).
Then the story goes back to present.
