James is a 12-year-old boy from a troubled home in London with a natural aptitude for math and a tendency to get into physical altercations. After a family tragedy, he is placed in the foster care system before being recruited by CHERUB, an organization that trains children to be spies. He is initially hot-headed and resistant to discipline but demonstrates resilience when pushed to his limits.

Lauren is James's younger half-sister who is sent to live with her abusive father following the sudden death of their mother. She is highly resourceful and possesses a keen observational eye. When faced with challenges, she often bypasses traditional methods to think outside the box. Her separation from James serves as a primary motivation for his early actions.

Kerry is a dedicated CHERUB operative who has been with the organization since she was six years old. She is repeating the punishing 100-day basic training regimen after a knee injury forced her to fail the previous year. She is fiercely determined and willing to push her physical limits to succeed alongside her new partner.

Amy is an older, experienced CHERUB operative who initially serves as James's swimming instructor. She utilizes a patient but firm approach to help him overcome his intense fear of the water. As a senior agent, she handles behind-the-scenes planning and partners with younger recruits on undercover assignments.

Kyle is James's roommate at the children's home who initially bonds with him over video games. He acts as a guide to James, transitioning him from the uncertain foster system into a highly classified environment. He possesses insider knowledge about the recruitment process and helps new arrivals adjust.

Dr. Terrence McAfferty, affectionately known as Mac, is the chairman of CHERUB. He oversees the recruitment and testing of potential child agents, maintaining a calm but commanding presence. He expertly evaluates unconventional strengths, seeing potential in rebellious behavior that others might punish.

Sebastian is a 10-year-old boy living at the Fort Harmony commune. Having been targeted and bullied heavily by outsiders for his off-the-grid lifestyle, he has developed a tough, aggressive exterior. He exhibits concerning behavior toward animals as a way to assert control in his environment.

Clark is an 11-year-old boy residing at the Fort Harmony commune. He mirrors his brother Sebastian's tough and sometimes cruel behavior, lashing out at others before they can hurt him. His worldview has been heavily shaped by his older brothers and the extreme ideologies present in his community.

Bruce is a young, energetic CHERUB agent who participates in field missions. He is tasked with causing calculated mayhem to create cover stories for more covert operations. He fully embraces the destructive nature of his assignments.

Joanna is a young girl who lives near the Fort Harmony commune. She is kind and offers a glimpse of normal, everyday friendship outside the intense demands of activism. She quickly recognizes the practical hardships of commune living and offers practical help to newcomers.

Fire is one of the older Dunn brothers residing at Fort Harmony. A former convict with extreme views on environmentalism, he holds significant influence over his younger brothers. He shapes their aggressive and anti-establishment perspectives on the world.

World is one of the older Dunn brothers residing at Fort Harmony. He shares the extreme, anti-establishment beliefs of his siblings and is involved in controversial activist activities. His rebellious nature deeply impacts the worldview of the younger children in his family.

Scargill is the third of the older Dunn brothers living at Fort Harmony. Like his older siblings, his past actions and radical environmentalist ideologies deeply impress the younger boys in his family. He represents the militant edge of the commune's activist wing.