Jack Reacher is a former US Army military police major who travels the country as a rootless drifter. Operating without a permanent address or a cell phone, he relies on his physical strength and deductive reasoning to survive. His strict personal code of justice frequently puts him at odds with official law enforcement and government agencies. He views bureaucratic institutions with deep suspicion, preferring to resolve issues directly and on his own terms.

Lila presents herself as a wealthy and elegant Ukrainian widow residing in London. She travels to New York claiming to seek an American soldier who assisted her mother in Berlin decades ago. Utilizing her beauty and a carefully constructed emotional backstory, she easily wins the trust of the police and those she hires. She proves to be highly intelligent and strategic in directing her private investigators.

John Sansom is a retired Army major, decorated Delta Force veteran, and current North Carolina congressman running for the US Senate. Despite his impressive public profile, his published political biography contains deliberate omissions regarding his covert military missions. Pragmatic and politically shrewd, he fiercely protects his public image from any threats tied to his clandestine past.

Svetlana is an older, non-English-speaking woman introduced as Lila Hoth's mother. Her quiet demeanor and aged appearance lend credibility to Lila's emotional backstory about searching for a lost benefactor from a past war. She remains close to Lila during their stay in New York, presenting a striking contrast to her glamorous companion.

Theresa Lee is an NYPD homicide detective assigned to investigate the incident on the 6 train. Initially pragmatic and focused on closing the case quickly according to standard procedure, she views Reacher with professional skepticism. As the investigation encounters aggressive interference from federal agencies, she finds herself reconsidering her strict adherence to the rules.

Jacob Mark is a New Jersey police officer and the grieving brother of the woman who dies on the subway. Refusing to accept the official narrative surrounding his sister's death, his conviction prompts Reacher to look deeper into the incident. He serves as an emotional anchor for the investigation, driven entirely by concern for his family's safety.

Susan Mark is a civilian clerk working for the US Army Human Resources Command at the Pentagon. Her erratic, terrified behavior during a train ride draws Reacher's attention and initiates the story's central conflict. Her personal vulnerabilities, including a strained relationship with her college-aged son, make her an easy target for extortion.

Operating initially under an alias, Springfield is a Delta Force veteran who serves as Congressman John Sansom's loyal head of security. He possesses a stoic, highly professional demeanor and is operationally savvy. He acts as the physical shield for Sansom's political ambitions, evaluating threats to his employer with cold efficiency.

This secretive team of federal agents aggressively monitors the fallout from the incident on the subway train. They refuse to provide identification, leveraging national security laws to enforce their authority. Their primary objective is suppressing information, showing little regard for local police procedures or individual rights.

Docherty is a detective with the NYPD and Theresa Lee's partner. He approaches the subway incident with a desire to quickly clear the case file, employing hostile questioning tactics when dealing with witnesses to expedite the process.

Elspeth is Congressman John Sansom's wife and a formidable political strategist in her own right. She acts decisively to protect her husband's Senate campaign, expertly managing inquiries into his past and setting traps for those she perceives as threats.

Peter Molina is Susan Mark's 22-year-old son and a star football player at the University of Southern California. He is reported missing after leaving a bar with an unknown older woman, escalating his family's fears and providing the leverage used against his mother.

Leonid is a large, physically imposing man employed by Lila Hoth. He is assigned to watch transit hubs and intercept individuals who might interfere with his employer's goals, relying on brute force to handle problems.