The Villainess and the Demon Knight (Manga) Vol. 1

When noblewoman Cecilia realizes she's been reincarnated into an otome game, her shock is further complicated by the fact that she's the villainess! What's worse, she can't seem to change her fate. Now, her engagement is broken off, the heroine is harassing her, and she's forced to work in a brothel--where her very first customer turns out to be the debauched Leader of the Imperial Guard, Lucas Herbst. This dangerous Casanova looks like he'll be the end of her, until he takes her as a fiancee!