There’s No Friendship Between the Grand Duke and the Marquis

With her twin brother sick on the day of the Oath of Allegiance ceremony, Obet Alphone attends in his stead dressed as him. However, at the ceremony, she meets a rude boy who mistakes her for a beggar. So she teaches him a lesson, after all, she'll probably never see him again. As fate would have it, she finds "that jerk from yesterday" in her drawing room...!