Princess Hilda is not named in the book, only referred to as "the elderly princess of High Norland". She is one of the princesses kidnapped by the djinn Hasruel. She plays a minor role, along with the characters from the first book.
She appears in the book, which is set two years after the second book, again in a minor role. It is mentioned in a letter from the King that she is to forgo her work in the royal library due to the visit by Sophie Hatter, Howl and Morgan. She has a role at the end of the book, where she discovers (along with her father) where the missing money of the Royal Family was.