Jack's imaginative and adventurous younger sister, who is seven years old, has blond pigtails and blue eyes. Her bold actions often drive the plot forward as she embraces the magic of the tree house.

An enigmatic and gallant medieval figure who appears on a black horse at a crucial moment to aid Jack and Annie. His mysterious presence adds to the story's sense of adventure and fantasy.

Three castle guards humorously nicknamed by the narrator based on their squinty eyes, red face, and long mustache, respectively. They play an antagonistic role by attempting to capture Jack and Annie.