I Committed a One-Night Mistake with the Young Lady?

A modern woman reincarnates as a novice pharmacist in a novel where the saint is destined to defeat a villainous noble. Unfortunately, she ends up working for the villain's household. But when she meets a delicate and beautiful "young lady" instead of the feared villain, she decides to protect and bring happiness to her. What she doesn't realize is that this "lady" is actually the villain himself in disguise.