Dewey is an 11-year-old math and mechanical engineering prodigy with a mismatched shoe due to a childhood injury. Highly independent but socially isolated, she loves reading The Boy Mechanic and building radios from scavenged parts. She travels to Los Alamos, New Mexico, to reunite with her absentee father and adjust to a secretive new environment populated by the world's brightest scientists.

Suze is an 11-year-old girl living at the Los Alamos facility with her scientist parents. Nicknamed "Truck" by the popular girls due to her size and assertiveness, she longs for social inclusion but struggles to fit in with her peers. She frequently turns to art, drawing, and comic books to occupy herself while her parents work long hours on the top-secret project.

Jim is a brilliant mathematician and loving, albeit absent, father working on a top-secret government project called the "gadget." His demanding work requires him to travel frequently and leaves him exhausted, yet he maintains a deep, affectionate bond with his daughter. He passes on his reverence for science and math to Dewey, providing the central stabilizing force in her life.

Terry is a chemist and former Berkeley professor working intense hours at the Los Alamos facility. As a compassionate mother and dedicated professional, she tries to balance the massive demands of her top-secret war work with caring for her daughter, Suze. She readily volunteers to look after Dewey when Jim Kerrigan is called away, providing maternal warmth to the displaced girl.

Philip is a metallurgist and former university professor who relocated his family to Los Alamos to assist with the Manhattan Project. He is deeply immersed in his daily work, viewing the scientific advancements of the gadget as a patriotic necessity to win the war, a focus that frequently keeps him away from his wife and daughter.

Barbara is a Girl Scout and one of the popular girls at the Los Alamos school. She is highly conscious of social status and actively participates in excluding both Suze and Dewey from her friend group, often mocking Dewey's physical differences.

Betty is part of the popular trio of girls at the school on the Hill. Along with her friends, she actively marginalizes those she deems different, physically shutting Suze out of social spaces to maintain her own standing.

Joyce is a member of the popular clique at the Los Alamos school who takes particular pleasure in bullying. She leads much of the verbal teasing directed at both Suze and Dewey, ensuring that the social hierarchy remains intact.

Charlie is a schoolmate of Dewey's who spends his free time constructing a boys-only treehouse with his younger brother. Unlike the popular clique at the school, he is friendly and accepting of Dewey's eccentricities, recognizing her as a kind person and defending her from the bullies.

Jack is Charlie's younger brother and a fellow resident of the Hill. He helps build a boys-only treehouse but is willing to bend the rules to include Dewey, demonstrating an open-minded attitude compared to most kids in town.

Dick is a young, friendly man with a deep knowledge of mechanics and radios. He encounters Dewey on her journey to Los Alamos and helps her with her radio project, later revealing himself to be a colleague of her father's at the secret facility.

Referred to informally as "Oppie," he is the scientific director in charge of the Manhattan Project at the Hill. He manages the brilliant, high-strung scientists living on the mesa and coordinates their efforts to build the top-secret gadget.

General Groves is the military authority overseeing the Manhattan Project. He operates from Washington, DC, and summons top scientists to discuss the progress of the top-secret gadget and evaluate foreign scientific papers.

Carmelita is the cleaner for the Gordon family at their Los Alamos apartment. She speaks Spanish and represents the local New Mexican workforce employed at the top-secret facility.

Mrs. Kovack is the neighbor of Dewey's grandmother. She temporarily looks after Dewey before the army escort arrives to take the young girl to New Mexico, showing little warmth toward her young charge.