Sapphire

The young Prince of Silverland, Sapphire is in reality a girl, but must pretend to be a boy in order to inherit the throne and prevent the evil Duke Duralumon from taking power. Her personality is more refined than tomboyish, but she's good with a sword and at night fights crime as the masked Phantom Knight. She's good-hearted, has a strong sense of justice and is competent and level-headed as well. Although most of her time is spent as Prince Sapphire, later in the series she is able to go about as a girl (wearing a blonde wig) in the persona of the Princess, Prince Sapphire's sister. In this way she is able to court Prince Frank.

(Source: Wikipedia)