Marie is a young single mother working in London while her daughter stays in the countryside to avoid the bombings. Because of her fluent French and childhood summers in Brittany, she is recruited by the British Special Operations Executive (SOE) to train as a wireless radio operator. She struggles with self-doubt but finds motivation in her desire to build a safer world for her child.

Eleanor is a Polish-Belarusian immigrant working as an assistant to the Director of the SOE in London. Driven by past family trauma, she convinces her superiors to establish a covert unit of female agents who can blend into French society. She assumes responsibility for recruiting, training, and managing these women. She maintains a brusque, professional exterior but worries constantly about the safety of her recruits.

Grace is a young widow living in New York City in 1946, working as an assistant for an immigration lawyer. She struggles with grief following the sudden death of her husband. Her discovery of a suitcase filled with photographs of unidentified young women sparks her curiosity. She decides to investigate the origins of the photographs, seeking answers about the women's identities.

Hans Kriegler is the head of the German intelligence agency operating in the region of France where the SOE Vesper network is based. He utilizes severe tactics to eliminate resistance movements and intercept Allied communications.

Josie is a seventeen-year-old SOE trainee who was orphaned after her parents died in a fire. She possesses natural athleticism and a spirited personality, quickly adapting to the rigorous demands of spycraft. She befriends Marie at the training facility in Scotland, offering both practical help and emotional encouragement.

Julian operates under the code name Vesper and commands the SOE network in northern France. He coordinates sabotage efforts, manages supply drops, and oversees incoming agents. The physical toll of the war is evident in his trembling hands, caused by nerve damage. He maintains a harsh exterior to protect his network but gradually softens around Marie.

The Director leads the Special Operations Executive in London, reporting directly to high-level government officials. He authorizes Eleanor's plan to recruit female agents and places her in charge of the operation. He acts as a pragmatic leader who prioritizes the broader war effort over individual concerns, often ignoring Eleanor's anxieties regarding field operations.

Mark is a lawyer working in Washington, D.C., and the former college roommate of Grace's late husband. He possesses connections within the government and the War Department. When Grace shares her discovery of the photographs, he uses his legal background and clearance to help her search for answers in military archives.

Tess is Marie's young daughter. To protect her from the London bombings, she lives with her great-aunt Hazel in the rural countryside. She serves as Marie's primary motivation for joining the SOE, as Marie hopes to secure a safer future for her child.

Aunt Hazel is Marie's relative who lives in the English countryside. She takes in young Tess during the London bombing blitz, providing a safe and stable home environment while Marie works in the city and subsequently deploys to France.

Will is a young pilot working with the SOE and the French resistance. He transports agents across the English Channel and assists with operations on the ground. As Julian's cousin, he shares a deep familial bond with the network commander but frequently debates him regarding tactics and local safety.

Tom is Grace's late husband. He died just after graduating from officer's training, before deploying to Europe with the American military. His death casts a long shadow over Grace's life, prompting her move to New York City.

Frankie is an immigration lawyer in New York City and Grace's employer. He assists refugees arriving from war-torn Europe, demonstrating compassion by providing personal financial help to clients who need housing.

Sammy is a young refugee from eastern Europe who survived the Vesterbork transit camp in the Netherlands. He seeks legal assistance from Bleeker & Sons to establish a life in America alongside his older cousin.

Richard is Marie's estranged husband. He abandoned her shortly after their daughter Tess was born. Marie initially lies to the SOE, claiming he died in combat, to avoid the stigma of his abandonment.

Raquel is the sister of Mark's friend. She works at the Pentagon and uses her access to help Mark and Grace view restricted military files concerning female operatives from the SOE.

Sally is one of the female SOE agents whose files Grace uncovers. She provides Grace with a crucial lead, as her file contains contact information for her surviving sister in Maryland.

Annie is Sally Rider's sister. During the war, she worked as a clerk for the SOE in London. She meets with Grace and shares her belief that the female agents in France were compromised by someone within their own organization.

Lisette owns a brothel in Paris that doubles as an SOE safehouse. She provides shelter for operatives passing through the city and utilizes her community connections to make discreet inquiries on their behalf.

Henri is a former member of the French resistance who now works at a government ministry. Recognizing Eleanor's name when she makes inquiries in Paris, he assists her by providing access to the former German intelligence headquarters.

Mick is a US Army Major and a former investigator of Nazi war crimes. He is stationed in Germany to prepare for upcoming tribunals and initially prevents Eleanor from accessing high-profile prisoners, though he eventually respects her determination.

Grace's mother visits New York City to deliver a check from Tom's estate and to encourage her daughter to return to her hometown. She worries about Grace's isolation and unconventional life choices.