Jack is an eight-year-old boy living in Frog Creek, Pennsylvania. Characterized by his methodical nature, he loves facts, reading, and taking careful notes in his trusty notebook. He is naturally cautious and often hesitant to embrace danger, relying on a red backpack filled with reference books to help him understand his surroundings. His logical approach balances his younger sister's impulsivity during their magical adventures.

Annie is a boisterous, imaginative seven-year-old girl with blond pigtails who loves make-believe. She is bold and impulsive, frequently charging ahead into magical environments without hesitation. Her daring nature often propels the siblings into action, as she prefers learning through direct experience rather than reading about it.

The Knight is a mysterious and gallant figure clad in shining armor. Operating in silence with a face entirely hidden by a metal visor, he embodies the romanticized ideals of medieval chivalry and heroism. His majestic presence simultaneously inspires awe in the children and provides critical assistance when they are lost on the castle grounds.

Red is a castle guard tasked with defending the medieval stronghold from intruders. Earning his nickname from his very red face, he is threatening and stern, attempting to imprison the children in a damp, clammy dungeon. Despite his intimidating physical demeanor, he proves highly superstitious.

Mustache is a medieval castle guard identified by his prominent facial hair. Paranoid about foreign enemies, he marches the children toward the dungeon while interrogating them about their origins. Like his companions, he is easily frightened by modern technology he does not understand.

Squinty is a castle guard recognizable by his squinting eyes. Tasked with protecting the duke's stronghold, he aggressively interrogates the children and helps drag them to the cold dungeon. His lack of understanding of tools from the future allows the siblings to escape his custody.