Florian is a 12-year-old middle school student who recently moved to Washington, DC, from Rome. He uses a keen system of observational skills called TOAST (the Theory of All Small Things) to notice details others miss. Though he struggles with typical middle school social situations, his exceptional deductive abilities earn him a position as a covert consulting asset for the FBI.

Margaret is Florian's confident, athletic, and highly observant best friend. She is a star player for her Washington Area Girls Soccer team and quickly learns how to use Florian's TOAST method to assist in solving local mysteries. As an adopted child left at a firehouse as a newborn, she maintains a strong desire to uncover the identities of her birth parents.

Special Agent Rivers is a calm and composed member of the FBI's art crime division. With a degree from Harvard and a PhD from Georgetown, he is analytical and quickly recognizes the practical value of Florian's observational skills. He serves as an essential mentor, giving Florian access to FBI training at Quantico while trying to protect him from the physical dangers of criminal investigations.

Known as "Nic the Knife," Nevrescu is a notorious Romanian American crime boss operating in the Washington, DC, area. He runs corrupt construction companies and launders money for the Eastern European League. The FBI actively investigates him, suspecting he has connections to the high-profile art theft at the National Gallery.

Hobbes is an impatient and dismissive British representative for the museum's insurance company. He frequently clashes with the security team and is quick to assign blame to others for the museum's vulnerabilities. He is particularly abrasive toward Florian, viewing the boy's presence at the crime scene as a waste of time.

Admiral Douglas is the tall, Southern-accented head of the FBI. After witnessing Florian's exceptional deduction skills on an unrelated case involving espionage, he officially offers the 12-year-old a position as an FBI consultant, establishing a college fund to compensate him for his services.

Jim is Florian's father, an American security systems designer whose work involves creating safeguards for major institutions around the world. He is highly supportive of Florian's unique talents, and his professional work provided the foundational inspiration for the boy's TOAST method.

Francesca is Florian's Italian-born mother and a skilled art conservator at the National Gallery. She supports Florian's unconventional hobbies and FBI involvement but firmly insists he maintain his schoolwork and personal life alongside his consulting duties.

Agent Cross is a petite, highly skilled FBI trainer from Philadelphia. Despite her mild appearance, she easily overpowers larger opponents and teaches Florian essential self-defense tactics and hostage preparedness during his time at Quantico.

Pavel Novak is an artist from the Czech Republic with ties to the Eastern European League crime syndicate. He operates as a copyist in the National Gallery, subtly altering his appearance while reproducing famous paintings in the museum's impressionism section.

Captain Abraham is a veteran firefighter at "The Hornet's Nest" firehouse. He is part of the crew that took Margaret in when she was abandoned as a newborn, and he maintains a protective fondness for her, offering clues about the night she arrived.

Serena is the director of museum security at the National Gallery of Art and a personal friend of the Bates family. The art theft occurs under her watch, putting her department under intense scrutiny from both the FBI and the insurance company.

Earl is a security worker at the National Gallery who assists the investigation by brainstorming with Florian. He helps the young detective evaluate the dimensions and characteristics of the stolen paintings.

Dr. Gonzalez is an eccentric forensic scientist who analyzes paint samples for the FBI. He operates through a barter system, trading quick test results for baseball tickets that he then donates to a local youth club.

Martin Kellogg is an FBI agent and a specialist on the activities of Nicolae Nevrescu. He provides Florian and Rivers with critical background information regarding the crime boss's operations and history.

Agent Crosby works in the Jewelry and Gems Division of the FBI. He is initially annoyed by Florian's presence but is stunned when the boy casually solves one of his espionage cases involving Chinese counter-intelligence by analyzing takeout menus.