The Princess of Pure Reason is a princess that was stuck in the Castle in the Air. She was freed by Milo when he passed through the Mountains of Ignorance. Her sister is the The Princess of Sweet Rhyme. They were adopted by King Azaz and the Mathemagician's father. They were in the Castle of Air when they couldn't solve their argument. In the film, she is yellow with a ponytail and has no face like the Terrible Trivium.