GIMME SOME SPOILERS!! THIS IS SO BEAUTIFUL
Despite having said she doesn't believe in marriage and therefore reassuring her sister, Charlotte, that they would be together forever, Kara falls in love with one of the Duke's men. She hides it from her sister because she knows it will worry/upset her, but Charlotte eventually finds out the same time the Duke does when Kara goes to meet her lover in secret.
...or it might have been one of the emperor's men, not the duke's. I confuse them a lost because they look similar and I'm not very good with korean.
Here's the raws if you want em:
https://tkor.camp/%EB%AA%85%EB%9E%91%EC%9E%90%EB%A7%A4%EB%B0%B1%EC%84%9C
Where is my banana milk was so hilarious