Prince Justin is a minor character who appears in the books and movie. In the books, he is the brother of the King of Ingary, while in the movie, he is the heir to the throne of the neighboring kingdom. He is described by his brother as a brilliant general, and the King does not feel safe going to war without at least either he or his Royal Wizard. He has a tendency to be cursed.
Simply referred to as "The Prince," he is the prince of a neighboring kingdom to Ingary who was cursed into existing as a turnip-headed scarecrow , with a tendency to get caught upside-down and helpless. The kingdom thought the Prince had been kidnapped by Ingarians, and declared war on them. Though Howl Jenkins Pendragon and the Witch of the Waste can see through the spell, his true identity is only revealed to Sophie Hatter and Markl at the end, when a kiss from Sophie breaks the curse. He appears to have fallen in love with Sophie, though she is in love with Howl. Free of his curse, he returns to his home to help negotiate a truce with Ingary.
Some time before the beginning of the story, Prince Justine goes to The Waste to search for the missing Wizard Suliman, who had been creating a garden at its border. The finding spell he has brings him to the Chipping Valley, and into Upper Folding, all the way to Howl's Castle. However, the spell begins pointing in two directions, and he gets lost. Asking Wizard Howl for directions was the last time he was seen before he disappeared at the edge of the Waste.
The Witch of the Waste captured him here, and used parts of his body for her creation, combined with Wizard Suliman. The leftover parts are used to create a servant with little memory who follows her around called Gaston. However, when he disobeys her in the hat shop a few days after May Day, she transforms him into a dog and leaves him tied in Market Chipping. He is later freed by Sophie Hatter on her exodus from Market Chipping, and runs away into the hills to Mrs. Fairfax's house where he remembered stopping as Prince Justin.
While it was difficult, the dog-man, now named Percival, was able to transform back into a person long enough to ask Lettie Hatter for help. For some time, he stayed with Lettie and Mrs. Fairfax, changing every so often back into a person. Every time, he would return to a different kind of dog, and it was becoming more and more difficult to change, and he could stay a man for less and less time.
Eventually, he is sent to look after Sophie when Lettie learns she is now living with Howl. He asks her not to tell Howl that he is a cursed man, and becomes part of the family for some time, living with them as they go into hiding.
Because of an imminent war with Strangia, the King of Ingary names Howl as his Royal Wizard to replace Wizard Suliman and orders him to search for Prince Justin, which he had already been doing in his own way since his disappearance.
Howl of course eventually discovers the cursed dog-man, and becomes angry with Sophie for not telling him. Lifting the curse, Percival is now himself again. Though he claims he does not remember much, when he is alone with Sophie, he explains that he worked for a bit with the Witch of the Waste, that he was there the day Sophie was cursed, and that he had been staying with Sophie's sister. However, he still does not remember anything beyond coming into service with the Witch of the Waste as Gaston.
Once the Witch of the Waste and her Fire Demon were destroyed, Howl is able to separate the bits of Prince Justin from Percival and the body the Witch had been creating to recreate him. The Prince returns after the story to the King of Ingary, his brother, to report on what has happened and to help him prepare for the coming war.
- While never named, it is likely that the "neighboring kingdom" the Prince comes from in the movie is really Strangia, which lies along the border and is at odds with Ingary, eventually going to war with them.