Griffin is an 11-year-old sixth-grader in Cedarville, New York, widely known among his peers as "The Man With The Plan." He possesses a strong sense of determination and a drive to solve his family's financial issues, which often pushes him to create elaborate schemes. Deeply frustrated when adults ignore or underestimate him, he feels a personal responsibility to take matters into his own hands.

Ben is Griffin's best friend and acts as the cautious voice of reason against Griffin's impulsive plans. He relies heavily on Griffin's strong personality but holds his own ground when he feels a plan crosses ethical lines. He keeps his medical condition, narcolepsy, a secret from his friends to avoid holding them back.

S. Wendell Palomino, often referred to by the neighborhood kids as "Swindle," is the short, stocky, and deceptively young owner of Palomino's Emporium of Collectibles and Memorabilia. He wears thick glasses that make his eyes look cartoonishly round and values money over the joy of the items he sells. He willingly cheats a child out of a valuable collectible to secure a massive profit for himself.

Darren is a school bully who delights in mocking Griffin and his family. He overhears the details of the heist and uses this information to blackmail his way into the operation. He shares the antagonist's greedy mindset, consistently looking out for his own personal gain over the success of the team.

Savannah is a deeply empathetic student who treats her family's numerous animals as equal partners rather than pets. She believes there is good in every animal and refuses to accept that any creature is naturally aggressive. Her moral compass causes her to hesitate regarding Griffin's illegal scheme, but she agrees primarily to ensure the safety of the dogs involved.

Antonia is a fearless and highly athletic student with a passion for rock climbing. She is a reliable and steady team player who teaches the rest of the group how to climb and rappel. She remains cool under pressure, taking on significant physical risks to ensure the success of the group's mission.

Logan is a child actor with an initially inflated ego and a dramatic flair. He takes his craft very seriously and uses his acting skills to play the role of a lonely new kid to distract a nosy neighbor. Through this assignment, he discovers a genuine connection and proves to be a humorous and sympathetic member of the team.

Melissa is a painfully shy computer whiz who usually hides her face behind a curtain of hair. She agrees to join Griffin's team simply because she is thrilled to finally be included in a group activity. When given the chance to utilize her technical skills, she becomes visibly animated and confident, and proves essential for disabling complex security systems.

Mr. Bing is Griffin's father, an aspiring inventor who recently quit his job to develop the SmartPick, an agricultural tool designed to harvest fruit without bruising it. His career change causes significant financial strain on the family, which forces them to list their house for sale. He remains largely unaware of his son's illicit activities to save the family home.

Mrs. Bing is Griffin's mother, who struggles with the financial instability brought on by her husband's new career as an inventor. She spends time taking Griffin and Ben to parks and malls to keep them out of the house while the realtor shows the property to prospective buyers.

Luthor is a large, exceptionally fierce Doberman pinscher used as a guard dog at Palomino's Emporium of Collectibles and Memorabilia. Trained to be highly aggressive, he presents a major physical obstacle to the store's security. He requires specialized handling to calm down and bypass safely.

Mr. Mulroney is an elderly, observant neighbor who spends his days and nights watching the street from his porch. His constant vigilance makes him a significant obstacle to any covert operations in the neighborhood. He enjoys playing backgammon and proves to be a lonely but sharp-eyed presence.

Mr. Martinez is Griffin and Ben's schoolteacher. He unwittingly aids Griffin's plot by assuming the boys are researching house blueprints for a creative writing assignment, directing them to the town hall for resources.

Mr. Slovak is Ben's father. He provides crucial assistance to the boys when they attempt to decipher the keypad passcode for the Emporium's security system.

The assistant manager is a tall, cadaverous employee at Palomino's Emporium of Collectibles and Memorabilia. He handles the physical labor of closing the store, including dragging heavy items inside at the end of the workday.