The Duke is having feelings for both Antonio and Rosalia not knowing they are same person
and the Marquis is really
and the Marquis is really
Spoiler!
The elder brother of Rosalia. So a small spoiler which might happen in future but don't know when. Duke fell for her as antonio just was denying it but when he found out about her being a woman I think he started seducing her and she fell in love with him in chapter 84 of the novel they did snu snu and in chapter 85 a rumor started because they did it while she was disguised as antonio that duke is gay.
I like to post spoilers for the stories I find interesting because I don't have much time to read novel due to busy schedule
you will find me in most romantic historical manhwa comment sections 
you will find me in most romantic historical manhwa comment sections 
