Reynie is an average-looking but highly intelligent twelve-year-old boy. He possesses strong observation skills and high emotional intelligence, making him a natural leader. Having been recently adopted by his former tutor, he enjoys a stable home life but struggles with a lingering distrust of strangers after his previous encounters with dangerous foes.

Sticky is a twelve-year-old boy with dark skin, glasses, and a shaved head. He possesses an extraordinary photographic memory that allows him to recall almost everything he reads. He is currently working on repairing his relationship with his parents, who coddle him after previously putting too much academic pressure on him.

Kate is a tall, confident, and highly athletic thirteen-year-old girl. She spent time working in a circus before moving to a farm with her father, a secret agent. She is fiercely protective of her friends and always carries a red bucket filled with practical tools.

Constance is a highly intelligent three-year-old girl. Because of her young age, she is prone to severe tantrums, stubbornness, and a frequent need for naps. She possesses an uncanny intuition that allows her to recognize hidden patterns, an ability that proves vital during the team's adventure.

Mr. Benedict is a brilliant, white-haired man who originally brought the children together to form their society. He has narcolepsy that is triggered by strong positive emotions like laughter. He believes strongly in seeing the good in others and is searching for a rare plant to cure his condition.

Ledroptha Curtain is an evil genius who seeks to dominate the world. Like his twin brother, he has narcolepsy, but his condition is triggered by anger, leading him to frequently use a wheelchair to avoid falling over. He commands a network of dangerous agents to enact his schemes.

Milligan is a highly skilled secret agent and protective father. Having recently regained his memories, he now lives a quiet farm life but continues to investigate threats to his family. He attempts to keep his daughter and her friends out of danger, often tracking them to intervene when they face lethal threats.

Number Two is one of Mr. Benedict's adopted daughters and dedicated assistants. She resembles a pencil in appearance and requires very little sleep. To compensate for her lack of rest, she must constantly consume food to maintain her energy levels.

Rhonda is an adult woman who serves as an assistant and adopted daughter to Mr. Benedict. She has a photographic memory similar to Sticky's. When her family members go missing, she stays behind to coordinate communication and handle the government officials investigating the disappearance.

Miss Perumal is Reynie's adoptive mother and former tutor from the orphanage. She provides Reynie with his first stable, loving home environment and cares deeply about his academic and personal growth.

Pati is Miss Perumal's mother and Reynie's adoptive grandmother. She lives with them in the suburbs outside Stonetown, helping to provide a warm family environment.

Mr. Washington is Sticky's father. He is currently struggling financially but remains deeply devoted to repairing his relationship with his son after previously pushing him too hard academically.

Mrs. Washington is Sticky's mother. She is highly protective of her son, overcompensating for past mistakes by coddling him as they rebuild their family dynamic.

Moocho is a former circus strongman who now works as a farmhand. He acts as a guardian for the property when the owner is away and causes a helpful distraction during a critical moment.

Captain Noland is a Navy veteran who commands the Shortcut, a fast cargo ship. He is an old friend of Mr. Benedict, who saved his life several times. He honors this debt by providing the children with passage and clues for their journey.

Joe "Cannonball" Shooter is the friendly third officer of the Shortcut. He is bombastic and helpful, managing the safety of the young passengers while dealing with the demands of corporate executives onboard.

McCracken is the formidable leader of the Ten Men, a group of businessmen-assassins who work for the antagonist. He uses modified office supplies as lethal weapons and acts as a direct physical threat to anyone interfering with his employer's plans.

Martina is a teenage agent working for the antagonist. Despite her youth, she harbors a deep dislike for children and enjoys exercising authority over older enforcers. She is petty, vindictive, and eager to prove herself.

S.Q. is a clumsy, dimwitted, but fundamentally gentle assistant. He follows orders out of loyalty rather than malice, making him vulnerable to manipulation by those with higher intelligence or stronger wills.

Jackson is an operative working for the antagonist. He tracks the children through Europe, attempting to capture them before they can decipher their next clue.

Jillson is a field agent searching for the children in Lisbon. She works closely with her partner to secure locations and intercept the protagonists.

Madge is a highly trained peregrine falcon. She follows her owner across vast distances and provides crucial assistance by delivering messages and tracking enemies from the sky.

Mr. Pressius is a demanding executive transporting valuable items on the Shortcut. He is arrogant and insists on prioritizing his cargo over the comfort of the other passengers.

Risker is a boat captain who runs a water transport business. He knows the location of a remote island and is detained by enemies who want to sever the children's travel options.

Sophie is a helpful librarian in the Netherlands. Despite being shaken by an assault from mysterious men in suits, she bravely provides the children with historical documents crucial to their search.

Daatje is a hotel employee who acts suspiciously upon the children's arrival. She secretly makes a phone call to alert dangerous men about the guests' presence.

Mr. Schuyler is a library employee who becomes frightened and angry after a violent attack on the staff. He threatens to call the police on the protagonists.

Hubrecht is a hotel staff member who holds a room key for the children, demanding to see a specific pendant before granting them access.