Nuritasu ~The False Bride~

A girl raised as a boy. One day, her destiny dramatically changes—an unnamed boy who longs to be a knight performs chores as a servant for the historic Romagnolo Count family. On his 17th birthday, the boy experiences his first menstruation and is told by his mother that he is female. Although she was pregnant with the Count's child, the mother secretly raised the child as a boy to protect it. When the Count learns of the child's existence, he decides to use the child as a body double to protect his daughter, Mayleen, from an unfortunate engagement. At that moment, the child is given a name it never had for 17 years, and its life dramatically begins to move. The given name is "Nuritas." Its meaning is "nothingness."