This is not your typical drama, I won't even put it under the rom-com segment but The Last Empress has all the things you are looking for. The story is really interesting also.
Love in the Moonlight is a classic saeguk drama in which the FL disguises herself as an eunuch (Male royal servant) to the Crown Prince.
Moon Lovers: Scarlet Heart Ryeo another classic saeguk drama. not really servant - master role per se but the "title" of the FL throughout the drama changed a lot. At one part, she was a family member, then she was a servant, then she was something else again. BUT, this is worth the watch.
