Amanda is a grieving mother and widow whose life shatters following the kidnapping and murder of her six-year-old daughter, Jess, and the subsequent suicide of her husband, Luis. Plagued by intense trauma, she attends court-ordered therapy while channeling her despair into an obsessive desire for revenge against Wallace Crone. She eventually agrees to a murder swap with a woman she meets in her bereavement group, seeking a sense of justice the legal system failed to provide.

Ruth is a Manhattan real estate agent who survives a near-fatal home invasion that leaves her hospitalized and physically scarred. She attempts to manage the resulting post-traumatic stress alongside her husband, Scott. In Amanda's storyline, a woman operating under the names Wendy and Naomi attends a court-ordered bereavement group, bonding with Amanda over shared grief and proposing a violent plan to bypass the criminal justice system.

Scott is a successful Manhattan corporate lawyer and former district attorney clerk. He provides comfort and security for his wife, Ruth, but harbors intense guilt over leaving her alone on the night of her home invasion. Driven by childhood memories of being bullied and current feelings of powerlessness, he becomes desperate to protect his wife from further harm.

Farrow is a New York City police detective assigned to investigate violent crimes, including Jess White's murder and Ruth Gelman's home invasion. Known in his precinct as "St. Jude" for taking on seemingly unsolvable cases, he suffers from a painful lower lumbar disc problem that restricts his mobility. He feels a heavy, personal debt to the victims of the crimes he investigates.

Initially known only as "Mr. Blue Eyes," this unidentified serial killer possesses piercing blue eyes, a square jaw, and a terrifyingly raspy voice. He attacks Ruth in her home, leaving her with severe injuries. In Amanda's storyline, Billy Cameron is a man driving a black Escalade who crosses paths with Amanda, presenting himself as a retired Marine Corps captain and grieving widower searching for a manipulative woman from an online forum.

Crone is a registered sex offender and a Wall Street stockbroker who comes from a wealthy, influential family. He is the prime suspect in the kidnapping and murder of Jess White. He avoids formal prosecution after his father's expensive legal team pressures the district attorney to drop the charges, making him the target of Amanda's obsessive stalking.

Jess is Amanda and Luis White's six-year-old daughter. Her tragic kidnapping from a park and subsequent murder initiates the novel's central conflict. Her memory heavily influences Amanda's daily life, and she is closely associated with her favorite toy, Sparkles the fluffy unicorn.

Luis is Amanda's husband and the father of Jess. He was present at the park when Jess was kidnapped. The overwhelming grief and guilt over the incident drove him to take his own life just one week after his daughter's murder.

Hernandez is an investigator with the New York City Police Department and Detective Farrow's partner. She is patient with Farrow's chronic pain and often acts as a sounding board during their investigations. She harbors strong personal opinions about the justice system, admitting she understands the urge to take extralegal revenge against guilty perpetrators.

Frank Quinn is a wealthy self-employed consultant who lives alone in a large, well-furnished home with an extensive collection of espionage and cyber warfare books. Wendy identifies him as the man who murdered her daughter, setting him up as the target for Amanda in the murder swap.

Travers is a man staying at the same hotel as Ruth and Scott Gelman. Because he has blue eyes and casually uses the word "sweetheart," Ruth suffers a panic attack, firmly believing he is the man who broke into her home.

Dr. Marin is a psychiatrist who treats Ruth Gelman during her lengthy stay at a psychiatric facility. He attempts to help her manage her severe trauma by teaching her visualization techniques, such as locking her fears inside an imaginary oak chest.

Dan Puccini is a blue-eyed police officer whom Ruth spots on television during a press broadcast. In her traumatized state, she hallucinates that he speaks directly to her, fueling her ongoing terror.

Jack is the only person who visits Ruth during her time in the psychiatric facility. He assists her by providing fake identification documents and aliases after her release.