Cheoeumbuteo Yeogi Isseonneundeyo

One day, wallflower Kim Hye-Jin wakes up as her web novel counterpart, Maria, a maid who is ignored by... well, everyone. No one ever really notices her, as if she was a ghost, but Grand Duke Dietrich can't seem to take his eyes off her. Seeing his rival's interest in the girl, Prince Elias resolves to use her for his own advantage in the battle for the throne. As misunderstandings and mystery surrounding Maria grow, she's quickly going from invisible to intriguing to... potentially irresistible!

(Source: Tapas, edited)