Read The Duke's Servant Manga:
Lindsay is a top-secret spy about to take on her most difficult mission yet: to disguise herself as a man?! Her target is the Duke. Rich, mysterious, and… an infamous womanizer. Cutting her hair, drinking elixirs to hide her voice, as a servant she must infiltrate his manor and investigate the mysterious disappearance of lords and ladies in the area. However, when she meets the Duke for the first t more.. ime and he kisses her, is it her identity that’s compromised or her heart...