Peter is a seventeen-year-old student at Sterling High who endures severe bullying from his peers. Following years of torment, he brings weapons to his high school and opens fire on his classmates in a devastating nineteen-minute event. Raised in the shadow of his popular older brother, Peter struggles deeply with social isolation and feelings of inadequacy. His actions force the town of Sterling to confront its rigid social hierarchies and the cruelty of its student body.

Josie is a popular high school student who survives the mass shooting at Sterling High. Though outwardly successful and dating the most envied boy in school, she hides a stash of pills to use if she ever loses her social standing. She suppresses her complicated past with Peter Houghton to maintain her status among the popular crowd. Following the tragedy, she suffers from trauma-induced amnesia regarding the event.

Alex is a recently appointed superior court judge in neighboring Grafton County, having previously worked as a public defender. She approaches life with a cool, methodical demeanor, prioritizing her demanding career over personal matters. As a single mother, she struggles to connect emotionally with her teenage daughter. Her professional objectivity faces an immense test when the tragedy directly intersects with her family.

Patrick is the sole detective on the Sterling police force, accustomed to handling routine, low-stakes crimes. He acts as one of the primary responders during the school shooting, rushing into the building against protocol to apprehend the shooter. Carrying a strong desire to help victims, he balances his duty to investigate the crime with his growing personal connection to the affected families.

Lacy is a dedicated nurse-midwife who prides herself on her strong emotional connections with her patients. As Peter's mother, she wrestles with intense guilt and denial regarding her son's actions. She questions whether she ignored warning signs in her home while mourning the loss of her older son, seeking to salvage her relationship with Peter despite his crimes.

Jordan is an experienced defense attorney who recently moved to Sterling with his family to escape the pressures of violent legal cases. He reluctantly agrees to represent Peter, recognizing the teenager is facing intense media scrutiny. He builds a controversial defense strategy centered around the psychological effects of chronic bullying, working to present his client as a victim rather than a monster.

Matt is the most popular athlete at Sterling High and Josie's boyfriend. Outwardly confident and heavily envied, he acts as the primary enforcer of the school's social hierarchy. He regularly targets less popular students for severe ridicule. His relationship with Josie appears perfect to outsiders but contains dark elements of physical intimidation and control.

Courtney is a member of the popular clique at Sterling High and one of Josie's close friends. She is highly focused on maintaining her social standing and frequently engages in mean-spirited pranks against students lower on the social ladder. Her casual cruelty contributes significantly to the toxic environment at the school.

Lewis is Peter's father and a university professor whose academic work focuses on the economics of happiness. He struggles to reconcile his intellectual understanding of the world with the chaotic reality of his family's crisis. Having previously taught his son how to handle firearms, he faces deep internal conflict following the tragedy.

Selena is Jordan's wife and a crucial investigative partner in his legal defense work. She helps gather background information about the town's history and the students' social dynamics, providing Jordan with important psychological insights to build his case.

Drew is a popular student and athlete at Sterling High who actively participates in enforcing the school's rigid social hierarchy. Alongside his friends, he frequently targets vulnerable students for public humiliation, treating harassment as a normal part of the high school experience.

Diana is a prosecutor who recently left a demanding job in Boston to work in the small town, only to be confronted with a massive tragedy. Working for the attorney general's office, she aggressively pursues the case against the school shooter, aiming to prove premeditation and secure a conviction.

Joey is Peter's older, deceased brother, viewed by their parents as the family's golden child. His athletic success and social popularity created an impossible standard for his younger brother to meet. Despite his parents' idolization, Joey treated Peter poorly in front of his peers.

Logan is an ambitious attorney who prioritizes his career and public image above his personal obligations. He establishes strict boundaries that prevent him from taking an active role in his daughter's life, showing little interest in knowing her.

Whit is Alex's former boss at the New Hampshire public defender's office. He serves as a trusted mentor, encouraging her to pursue a judicial career and offering guidance as she balances her career aspirations with the complexities of her personal life.

Mr. McCabe is a mathematics teacher at Sterling High who perishes during the shooting. He attempts to offer guidance to struggling students and is one of the few faculty members who tries to intervene when observing troubling classroom dynamics involving Peter.