seal it with a kiss

finished reading this last night (absolutely devastated) but i was left still confused what the relationship between yuhan and ilha was. throughout the story, we saw him and ilha living together, sleeping in the same bed, and acting intimately, making us think he was cheating on myeong or that ilha was his real partner. but that was revealed to be false (?) because ilha was actually his deceased brother’s partner and this whole time, she was simply plotting his revenge against him. we also see in yuhan’s backstory how ilha cared for him, even worried that them living together as an adult man and woman would be wrong. she practically took him in, raised him, and even taught him to live a normal life by letting him work as a manager under her. so i’m confused how they got to the point of acting so intimate with each other, when even until yuhan tried to kill himself and found myeong’s pictures, their relationship seemed solely like a guardian and depender type. so what happened? was it part of her revenge to ruin yuhan because she wanted to ruin everything that man cherished? or did yuhan want to comfort her after what his brother did? i’m just so confused, i don’t think it was addressed or did i miss something?