Maisie is a private detective in her early thirties who uses psychological insights and keen observation in her practice. Originally from a working-class background, she began her career as a maid in the Compton household before earning an education and serving as a battlefield nurse during World War I. She is compassionate, highly intuitive, and deeply affected by her wartime experiences.

Lady Rowan is a wealthy aristocrat with progressive political views and a dedication to women's suffrage. When she catches her young maid Maisie reading in the family library, she decides to sponsor the girl's private education. She is a loyal patron who increasingly depends on Maisie for personal help regarding her own family.

Dr. Blanche is a brilliant polymath with expertise in medicine, forensic science, philosophy, and psychology. He is an old family friend of the Comptons who takes Maisie on as a private student. He guides her educational journey and eventually trains her in his highly psychological, observational methods of criminal investigation.

Frankie is Maisie's father, a costermonger who grieves deeply for his late wife. He loves his daughter fiercely but harbors significant anxiety about her moving into social spheres where he cannot follow. He eventually takes a job managing the Compton family's horses, finding a quieter life in the country.

Billy is a World War I veteran who works as a building caretaker in London. He remembers Maisie as the battlefield nurse who helped treat his severe leg injury in France. He is immensely practical, fiercely loyal, and eager to assist Maisie in her investigations, despite struggling with his own wartime traumas.

Adam Jenkins is the founder and leader of the Retreat, a remote, walled farm dedicated to sheltering gravely wounded WWI veterans. He demands strict adherence to community rules, including the elimination of residents' last names. He maintains a commanding, military-like authority over the vulnerable men in his care.

Enid is an ambitious and perceptive housemaid who acts as an early, somewhat cynical mentor to Maisie. She hopes to improve her social standing and speech, eventually taking a dangerous but high-paying job in a munitions factory during the war. She views the rigid class system as an obstacle to her personal happiness.

Simon is a dashing young surgeon from a well-to-do family. He meets Maisie shortly before leaving for the war, and their romance blossoms when they cross paths again at a military hospital in France. He is sensitive, dedicated to saving lives, and deeply affected by the wartime suffering he witnesses.

Priscilla is Maisie's wealthy, extroverted college roommate. She prioritizes a joyful social life over academics and pushes Maisie to experience life outside the library. When the war intensifies, her cheerful demeanor gives way to a sense of duty, and she leaves school to serve as an ambulance driver.

Christopher is a wealthy businessman and publisher who becomes Maisie's first independent client. He hires her because he suspects his wife is engaging in an extramarital affair during her unexplained absences. He is anxious and genuinely loves his wife, though he struggles to bridge the emotional distance between them.

Celia is the elegant wife of Christopher Davenham. She makes mysterious biweekly trips to a cemetery in Kent, carrying deep, unresolved grief from her past. Her secretive behavior and emotional struggles become the focal point of Maisie's first case.

Lord Julian is Lady Rowan's husband. While he supports his wife's charitable efforts primarily out of respect for her and Dr. Blanche, he initially doubts that a servant can truly thrive at Cambridge. As a government official, he possesses high-level contacts that prove useful.

James is the son of Lord Julian and Lady Rowan. Before the war, he is a boisterous young man engaged in a forbidden romance with a housemaid. After his military service, he returns home withdrawn and melancholic, seeking complete isolation from his aristocratic family.

Vincent is a deceased World War I veteran who suffered severe facial injuries during combat. After the war, he cut ties with his former life and family to live at the Retreat, where he eventually died under mysterious circumstances.

Iris is a fellow nurse who befriends Maisie during their deployment to the front lines in France. She provides essential emotional support and acts as a grounding, practical presence amid the horrific realities of their battlefield medical work.

Carter is the strict but fundamentally decent butler at the Compton estate. He is responsible for maintaining discipline among the household staff and holding them to high standards of presentation, though he shows quiet support for Maisie's unconventional education.

Jack is a cynical newspaper seller who operates near Maisie's London office. He initially assumes Maisie is a wealthy snob until she surprises him by responding in an authentic working-class accent, which quickly establishes a friendly neighborhood rapport between them.

Tom is a cemetery groundskeeper who lost his own son to the psychological aftermath of the war. He provides Maisie with crucial information about the unusual cluster of veterans buried in his graveyard, sparking her interest in the Retreat.