Taking place in a medieval fairy-tale setting, Princess Knight is the story of young Princess Sapphire who must pretend to be a male prince so she can inherit the throne (as women are not eligible to do so). This deception begins as soon as she is born, as her father the King announces his baby is a boy instead of a girl. The reason for this is that the next-in-line to the throne, Duke Duralumon, is an evil man who would repress the people if he were to become king, and because of this the King will go to any length to prevent him from taking over. 
(Source: Wikipedia)

Sapphire was supposed to be a tomboyish junior high school girl who's not interested in boys, but it all changed on her 14th birthday. Not only did she find out that she's prince Sapphire, the Ribbon Knight, but she also has to protect a country called Silver Land!?

Princess Sapphire is a girl raised as a Prince. Through the mischief of an angel, the princess is born with both a girl's body and boy's mind. Since there is no boy successor in her kingdom, Sapphire is raised as a boy, but evil ministers try to reveal her secret. Unable to put up with the kind of vicious conduct prevailing in the kingdom, Sapphire disguises herself as "Princess Knight" and wields her sword of justice. 
This is an animated version of the girls' manga featuring Sapphire's romance and adventure, which marked the first made-for-TV animated program geared towards girls in Japan. 
(Source: Official site)

Franz Charming