Dr. Laszlo Kreizler is a brilliant psychologist, known in the 19th century as an alienist. He operates the Kreizler Institute for Children and believes deeply in the theory of individual psychological context, which traces adult behavior back to childhood experiences. His unorthodox views make him a pariah among New York's elite and the traditional medical establishment. He leads a secret investigation into a series of child murders, relying on his unique psychological profiling methods.

John Schuyler Moore is a police reporter for the New York Times and the narrator of the story. He possesses a deep understanding of the city's corruption and maintains useful contacts throughout New York's underworld and police department. Though initially skeptical of some of Kreizler's psychological theories, he serves as a loyal investigative partner. He carries personal grief over his late brother, fueling his sympathy for the city's exploited children.

Sara Howard is the police commissioner's secretary and the first woman to work at New York City police headquarters. She aspires to become a female police detective, approaching her work with determination and carrying a .45-caliber revolver. Roosevelt appoints her as his personal liaison to Kreizler's investigation, where she functions as an equal member of the team. Her perspective proves essential to constructing accurate psychological profiles.

Theodore Roosevelt is the energetic president of the board of police commissioners. He actively purges the New York City police department of its corrupt old order and attempts to modernize the force. He authorizes and supports Kreizler's secret investigation despite the political risks, providing the team with resources and protection against his powerful enemies.

Japheth Dury, who later assumes the name John Beecham, is a former corporal discharged from the US Army for mental unfitness. He grew up in New Paltz, New York, where he endured a severely abusive and traumatic childhood under a cold mother. His background includes extreme isolation, a severe facial tic, and exposure to his father's gruesome war photographs. His traumatic history becomes a central focus of Kreizler's psychological research.

Lucius is the shorter of two Jewish detective brothers recently recruited by Roosevelt. Unconnected to the department's corrupt old guard, he brings fresh, modern ideas to the police force. He specializes in medical examinations and forensic analysis, eagerly applying new scientific methods to the investigation.

The taller of the Isaacson brothers, Marcus is a formidable scientific mind who excels at fieldwork. He utilizes cutting-edge forensic techniques, such as fingerprinting and handwriting analysis, to decipher physical evidence left at crime scenes. His sharp observational skills make him a valuable asset during surveillance and rooftop investigations.

Mary Palmer is Dr. Kreizler's housekeeper. She suffers from motor aphasia, making speech difficult, so she communicates primarily through gestures and single words. Kreizler rescued her from the penal system after she committed a violent act in response to severe childhood abuse. She remains deeply devoted to the alienist and his household.

Cyrus Montrose is an imposing man who serves as Kreizler's valet, carriage driver, and bodyguard. Like Kreizler's other servants, Cyrus has a troubled past tied to childhood trauma, specifically witnessing his parents' deaths during the 1863 draft riots. He provides vital protection and practical assistance to the investigative team throughout the city.

Stevie Taggert is an eleven-year-old boy who works as Kreizler's carriage driver and general errand boy. He possesses significant street smarts, having survived a difficult life in the city's underbelly before Kreizler took him into the institute. He uses his knowledge of rough neighborhoods to assist the investigators with surveillance.

Patrick Connor is a corrupt detective sergeant who embodies the brutal old guard of the New York City police department. He resents Roosevelt's reform efforts and frequently attempts to obstruct Kreizler's work. He relies on intimidation, violence, and bribery rather than genuine police work, acting as an enforcer for the city's entrenched power brokers.

Paul Kelly is an intelligent, refined, yet dangerous gangster who controls much of the illegal activity in the Five Points neighborhood. Unlike typical thugs, he possesses a sharp intellect and a keen understanding of the city's social dynamics. He manipulates immigrant mobs to suit his political aims and views the city's elite with calculating contempt.

Thomas Byrnes is the wealthy, corrupt former police inspector whom Roosevelt replaced. He maintains significant influence over the department's old guard and fiercely defends the established social order. He utilizes violence and intimidation to suppress investigations that threaten his authority.

J. P. Morgan is New York City's legendary financier. He views the city's social stability through an economic lens, remaining primarily concerned with preventing riots or socialist uprisings. While intimidating, he demonstrates a pragmatic willingness to hear unconventional ideas if they might restore public order.

James T. Ellison is the boorish, physically imposing owner of Paresis Hall, a corrupt pleasure club. He pays heavy bribes to the police to keep his illicit business running and relies on brute force to manage his establishment.

Georgio Santorelli is a thirteen-year-old boy who worked at Paresis Hall. His tragic death at the hands of a mutilator spurs Roosevelt to authorize Kreizler's secret investigation.

Joseph is a deeply traumatized boy, no older than ten, working at the Golden Rule Pleasure Club. His vulnerability strikes a personal chord with Moore, who attempts to help him escape his dangerous environment.

Jesse Pomeroy is a notorious prisoner who committed horrific acts of violence during his youth. He suffers from physical disfigurements, including a harelip and a milky eye, which severely impacted his childhood. Kreizler interviews him to gain insight into the psychological formation of violent individuals.

Jack McManus is a former prizefighter who currently works as a bodyguard for Paul Kelly. He serves as muscle for the gangster's various operations and intervenes during critical confrontations to protect his employer's interests.

Adam Dury is a struggling farmer who grew up in upstate New York. He harbors deep resentment toward his cold, abusive mother and remembers the severe trauma his family endured. He provides Kreizler with essential background information regarding his family's dark history.

Ali ibn-Ghazi is a fourteen-year-old son of Syrian immigrants who worked at the Golden Rule Pleasure Club. He is one of the victims whose death helps the investigators track the killer's movements.