I have read this one as well as kiss me liar and desire me if you can and I was wondering if there are any other comics in the same universe
This one https://www.mangago.me/read-manga/kiss_the_scumbag/
But it's not part of the two main series "...if you can" and "Kiss me..."
The rest was not adapted to manhwa so far, only available as webnovels/novels.
I sort of assumed that Ain was the duke when he was introduced there were quite a few time were I questioned this theory I also thought maybe he was the duke illegitimate brother or something like that but it seems like he's just a good actor
I feel like the reason all these characters are so fucked up is that the society in this universe as a whole is even more fucked up and these characters are byproducts of it