Maybe it really is Charin? I mean they both like each other currently and Jin is who Charin freely chose to love unlike Hyehan who forced drug dependency and pregnancy on him not to mention the verbal degradation and other crap he put him through. With Jin the feelings ARE mutual and Charins feelings for Jin show that despite the hard circumstances, the memory of Jin is what gets him through.
I think its an interesting comparison, with Hyehan being so powerful and getting everything he wants but what he wants the most is not something he can force. Hes forced to confront his limitations. Charin says it himself. Hyehan can control his body and circumstances but he cant make Charin love him.
The relationships in this manhwa are so complicated and love interests dont cleanly map one to one with sexual relationships and objectively significant shared experiences, which is probably part of why its so hard to tell. I mean look what happened with Kimoto and Garon.
I dunno, but I hope the author doesnt discontinue this manhwa and we get to find out what she has in mind for Jin

And I still don't know who the romantic interest is supposed to be for Jin