Just finished chap 21. This is what I understand so far (spoilers ahead): Yeonjin is obsessed with wanting to be just like his father (for revenge... not sure yet) due to his childhood. His wants and what he thinks his father would want is all blurred... up until I'm not even sure if he wants/likes Moonhyuk or if it's because he needs to like Moonhyuk (and have him like him) in order to be more like his father.
He pretends to be just like Moonhyuks previous lover to get Moonhyuk to like him... which I think Moonhyuk understands (doesn't understand why but knows his pretending to be his ex lover). Moonhyuk goes with it because he sees Yeonjin as his new master and would do anything for his master... this is what I understand so far. I do believe Yeonjin likes Moonhyuk but again, his want/like is a mixed between actually him and because it makes him more like his father. It's confusing af (shrugs).
Mania