Irene

It's so clear that a) Ilic is really in love with the Duke or b) he is just fucked up with stockholm syndrome. He blushes when the duke call his name, during or outside of sex. Describes the dude as his type without even realizing it and hasn't had sex with anyone else in the mean time (as far as we know at least). Feels slightly triumphant when he sees Sayan having eyes only for him.

There is no reason for him to be faithful to him since they aren't in a relationship. I don't think Sayan minds either because I am sure he knew he was having sex with Leiliya when he was outside the castle. He even offers Ilic the option to sleep with women himself. Sayan wants to bind Ilic to him but doesn't mind if he seeks out intimacy with other people.

Either way, he is attached to the Duke quite evidently. He no longer has a contract he needs to upheld with Sayan and could very well just kill him and get this over with and run away, live his life, marry a woman and start a family. He really doesn't gain anything by sticking around with the Duke anymore. It's hard to believe that he didn't have any opportunities to kill Sayan in these 12 long years, honestly.

Man, I hate when these backstories insert long ass detailed sex scenes between the MC and his ex. Like c'mon mama, just please tell us how they fell apart instead of inserting long sex scenes with the ex. It feels so weird.