Migawari Hakushaku Series

Mireille is the daughter of a bakery owner and her twin brother Fred was adopted into another family when they were kids. One day a letter from her brother arrived, saying he was in love with another man's woman. She replied, challenging him to be a man and claim the woman as his.

Two months later, a beautiful young man named Richard, claiming to be a messenger from Fred, visits and tells her "I've come for you." When the bewildered Mireille resists, he proceeds to kidnap her! It turns out that Fred has eloped with the crown prince's fiance, and it is now her responsibility to impersonate Fred - the former Count of Bernhardt!