The Fairy Princess is a character that appears in the Dexter's Laboratory IDW comic book miniseries (issues 3 and 4).
History
The Fairy Princess is the rightful ruler of the kingdom Dexter visits, having been overthrown by Dee Dee after she was wished out of existence by Dexter. Upon meeting Dexter, she asks him to help her take back her throne in exchange for Dee Dee gaining back her memories and returning to her own world.
She sends Dexter into the castle, disguised as a girl, but he is soon caught and taken to a girly "torture" chamber. After Dexter explains how much he truly cares for his sister, Dee Dee's memories are restored and she leaves her crown for the Fairy Princess before going home with Dexter.