Elizebeth is a pioneering cryptanalyst who starts her career investigating supposed literary ciphers before transitioning to breaking military and criminal codes. Her background in poetry and literature sharpens her sense of patterns in language, giving her an advantage in deciphering encrypted messages. Operating in a male-dominated environment, she is a methodical scientist who founds the US Coast Guard's first formal Cryptanalytic Unit.

William is an intelligent plant biologist who becomes a foundational figure in American cryptology. He develops statistical methods for codebreaking and designs secure machine systems for the government. His dedication to his classified work subjects him to immense pressure and periods of deep anxiety, forcing him to balance his patriotic duties with his fragile mental health.

George Fabyan is an eccentric textile magnate who funds the 350-acre Riverbank Laboratories. He cultivates a public image as a scientific visionary but exercises manipulative, authoritarian control over his employees. His obsession with proving Francis Bacon authored Shakespeare's plays drives him to recruit cryptologists, inadvertently sparking the birth of modern American codebreaking.

J. Edgar Hoover is the formidable director of the FBI, a master of public relations and bureaucratic maneuvering. He aggressively expands his agency's intelligence operations while lacking the specialized technical expertise required for cryptanalysis. He frequently utilizes the intelligence gathered by other departments to stage high-profile raids, prioritizing media attention over operational security.

Johannes, operating under the codename 'Sargo,' is an adaptable and pragmatic SS intelligence officer. He manages clandestine Nazi spy rings across South America, utilizing a variety of sophisticated ciphers and commercial airlines to transmit information to Germany. He is a dangerous adversary who learns from his mistakes and continuously upgrades his communication security.

Elizabeth is an educator and the chief proponent of the theory that Francis Bacon embedded secret messages within Shakespeare's texts. She uses a biliteral cipher method that relies on identifying minute typographical variations in historical prints. Her subjective, flawed approach ultimately forces her assistants to develop more rigorous, evidence-based codebreaking methods.

Herbert is a prominent cryptologist who runs the US Army's early codebreaking unit. He is a brash, publicity-seeking figure who views intelligence work through a different ethical lens than his more secretive colleagues. His willingness to publish a sensational tell-all memoir about his operations creates a public scandal and permanently alters government secrecy policies.

Joseph is a forward-thinking military officer in the US Army Signal Corps who recognizes the critical importance of signals intelligence. He serves as an institutional bridge, connecting civilian cryptographic talent with official government operations. His support provides legitimacy and essential resources for emerging codebreakers during a period of disorganized military intelligence.

Gustav, known as 'Luna,' is a former navy radio operator who runs a radio shop in Buenos Aires as a front for building clandestine transmitters. He is technically proficient and forms a highly effective partnership with the espionage network's leadership. He manages the complex radio circuits that link the South American spies directly to Berlin.

Albrecht is a capable professional operative who initially works for a rival intelligence service before joining the SS-led network. He helps establish courier systems and sets up illicit radio communications using a docked Swiss ship.

Leonard is a Coast Guard Lieutenant who takes official command of the codebreaking unit during wartime reorganization. He handles the bureaucratic and military duties of the department, though he relies entirely on civilian expertise for the actual intelligence work.

Frank is an innovative cryptanalyst who works closely with the Army's top codebreakers to dismantle complex foreign cipher machines. He possesses strong analytical skills and contributes significantly to the development of highly secure American cipher devices.

Genevieve is a focused cryptanalyst working on the Army's most challenging cipher machine problem. Her sharp eye for patterns leads to a historic breakthrough that allows the United States to read encrypted diplomatic traffic.

F. J. M. Stratton, nicknamed 'Chubby,' is a jovial former astronomer turned British intelligence officer. He invents specialized radio-locating equipment and helps establish an cooperative relationship between British listening posts and American analysts.

Barbara is the eldest child of the Friedmans. Growing up in a household deeply involved in secret intelligence work, she communicates with her parents through coded letters and develops an interest in political activism as a young adult in New York.

John Ramsay is the son of the country's top codebreakers. As a teenager during World War II, he attends prep school and makes plans to join the Army Air Corps, bringing the reality of the global conflict directly to his parents' doorstep.