Jakob is a 19-year-old mathematical prodigy working at the highly restricted Code and Cypher School at Bletchley Park. Reeling from the sudden heart attack of his father and the reported death of his mother in Poland, he buries his grief in his vital work breaking German codes. Bound by the Official Secrets Act, he is forced to keep his family and friends at an emotional distance. This silence causes a severe strain in his relationship with his younger sister.

Lizzie is a 14-year-old, fiercely observant girl who completely rejects the official story that her mother died in a bomb blast. Highly analytical and deeply stubborn, she uses her youth to her advantage, consistently outwitting adults who underestimate her. Instead of fleeing the war, she schemes her way into working as a messenger at Bletchley Park. She uses this position to stay near her brother and investigate her mother's mysterious disappearance.

Willa Novis is Jakob and Lizzie's American mother, who requested a transfer to the US Embassy in Poland shortly before the war broke out. Officially reported killed during the invasion, she leaves behind a legacy of unanswered questions. Her children remember her as loving and dedicated, but her mysterious final actions and a locked diary suggest she harbored significant secrets. Lizzie remains utterly convinced she is still alive.

Colin is the resolute, mechanically minded son of the local innkeepers in Bletchley. Unable to join the regular military due to a poorly healed broken leg, he eagerly serves in the Home Guard and supports the war effort from home. He becomes a crucial, grounded ally to Lizzie, helping her pick locks and explore the area while harboring his own anxieties about his older brother fighting in the Royal Air Force.

William Jarvis is a relentless MI5 counterintelligence officer assigned to investigate the Novis family. Deeply suspicious of Willa's activities in Poland, he believes she may have defected to the Nazis. He uses intimidation and constant surveillance to pressure Jakob and Lizzie, repeatedly threatening them with charges of treason if they obstruct his investigation.

The Colonel is the stern commanding officer overseeing the covert operations at Bletchley Park. Tasked with managing the brilliant but eccentric codebreakers, he enforces the Official Secrets Act with absolute authority. He assigns Lizzie a job as a messenger, unwittingly giving her access to the various compartmentalized sections of the estate.

Mr. Fleetwood is Gran's long-suffering assistant, tasked with the frustrating job of acting as Lizzie's chaperone. His primary mission is to safely escort Lizzie out of war-torn England to the United States. He frequently finds himself completely outmaneuvered by his much younger, highly evasive ward.

Marion is a young Austrian Jewish refugee who fled Vienna before the war, separated from her sister who was sent to Florida. Working as a messenger at Bletchley Park, she bonds quickly with Lizzie over their shared experiences of familial separation and the anxieties of wartime. She proves to be a deeply loyal and courageous friend.

Beryl is a sharp, quick-witted mathematician newly recruited to the Bletchley Park codebreaking team. She approaches the seemingly impossible mathematical odds of the Enigma machine with a blend of brilliant insight and dry humor. She quickly impresses her peers and catches Jakob's romantic attention.

John is a young mathematician brought in from Cambridge to assist in cracking German codes. Initially intimidated by the astronomical number of combinations the Enigma machine produces, he works closely with Jakob and Beryl to find exploitable patterns in the ciphers.

Based on the historical figure, Alan Turing is a brilliant and eccentric codebreaker working on a massive machine to decipher Enigma codes. His highly unusual habits, such as burying his savings in silver ingots in the woods to protect them from a potential invasion, attract the suspicion of the local children.

Gordon Welchman is a senior figure at Bletchley Park and Jakob's former Cambridge professor. He works tirelessly in Hut 6 to attack German Army and Air Force ciphers, providing both academic mentorship and strategic direction to the younger mathematicians under his wing.

Nigel is a young local boy who lives on the Bletchley property, as his father is the estate's maintenance man. Highly observant and naturally suspicious, he frequently shadows the adults around the estate, taking a particular interest in Alan Turing's eccentric behavior.

Olivia is a clerk at the US Embassy in London and a close friend of Willa Novis. She serves as a vital link to Willa's past, providing Lizzie with crucial details about her mother's final days in Warsaw before the evacuation.

Gran is Jakob and Lizzie's strict, imposing grandmother. Living in a large London mansion during the preparations for air raids, she is determined to ensure Lizzie's safety by forcibly sending her away to America, employing Mr. Fleetwood to carry out her demands.

Viola is the Novis family's former nanny who now serves as a housekeeper in Wales. She remains a trusted confidant for Lizzie, forwarding important mail and acting as a steady, comforting presence in the siblings' lives despite the physical distance between them.

Simon is Colin's older brother serving as a pilot in the Royal Air Force. Though he fights off-page, his dangerous military service casts a long shadow over his family at the inn, embodying the constant, terrifying reality of the war for the civilians waiting at home.

Mrs. Tilbury is Colin's mother and the kindhearted owner of The Shoulder of Mutton Inn. Despite strict wartime rationing, she generously shares her saved tea with her guests, maintaining a welcoming environment while silently enduring the agonizing wait for news of her eldest son fighting in the RAF.

Denny is Olivia's boyfriend in London. He assists the girls by driving them around the city on his motorcycle, helping them evade Mr. Fleetwood and the authorities when they need to make a quick escape from an embassy party.

Commander Bradley is an officer at Bletchley Park who assigns Lizzie her job as a messenger. He introduces her to Marion and integrates her into the park's operational staff, inadvertently placing her in a position to observe the entire facility.

Dilly Knox is a veteran codebreaker at Bletchley Park who previously worked alongside Jakob's late father. He provides Jakob with a sense of historical continuity and deep respect for the Polish mathematicians who laid the groundwork for the current Enigma project.