Emma is a 21-year-old immigrant living in Flint, Michigan. Having fled her home in Canada to escape a father who punished her simply for being born a girl, she strikes upon a bold idea to join the Union Army. She cuts her hair, dons male clothing, and successfully enlists as Private Franklin Thompson. Initially serving as a field nurse, her drive to actively fight for freedom pushes her into dangerous espionage work behind enemy lines.

Mrs. Butler is the maternal wife of a Union Army chaplain. She provides a comforting presence for homesick soldiers in the camp. When she learns the truth about the young field nurse, she reacts with surprising open-mindedness, offering practical help and emotional support for the dangerous missions ahead.

Living in St. John, Canada, this harsh man deeply resents having a daughter instead of a son. He subjects his child to constant verbal abuse and unreasonable physical expectations. His cruelty ultimately drives her away from home and fuels her lifelong need to prove her capability.

James is a young soldier from Boston who knew Emma before the war. His presence in the military and exposure to frontline combat forces her to confront the brutal, personal cost of the conflict, inspiring her to take greater risks for the Union.

The camp chaplain ministers to the spiritual needs of the regiment. Trusting his wife's judgment, he leverages his respected position to recommend a promising young candidate for sensitive, high-risk intelligence work.

General McClellan is a high-ranking commanding officer in the Union Army. He relies heavily on the detailed observations gathered by undercover scouts to plan large-scale troop movements and offensives into Virginia.

Dr. Hodes is a busy medical professional tasked with overseeing the Union field hospital. He manages the crowded medical tents and relies on his nurses to handle the overwhelming influx of casualties from nearby battles.

Allen is a young Confederate soldier suffering from a fatal case of typhoid fever. Separated from his unit and resting in an abandoned cabin, he relies on a passing stranger to deliver his pocket watch to his commanding officer.

Major McKee is an officer in the Confederate Army. He takes responsibility for the personal effects of his deceased troops and organizes burial details, occasionally accepting assistance from helpful civilians who pass through his camp.

Operating a business in Louisville, Kentucky, Mr. Aylesworth is a merchant who secretly supports the Confederacy. He uses his store and social position to quietly connect undercover agents and facilitate their travel.

Allen Hall is a Confederate agent tasked with transporting secret documents through neutral territory. He travels heavily armed and remains highly suspicious of strangers who attempt to join his nighttime journeys.

Rebel is a strong pony initially attached to the Confederate forces. After an unexpected change in ownership, the horse becomes a valuable asset for quick escapes across treacherous ground.