Bronwyn is a high-achieving mathlete who feels intense pressure to maintain perfect grades and follow her successful parents to Yale University. She is known as a strict rule-follower, making her presence in detention highly unusual. Beneath her squeaky-clean image, she is fiercely protective of her younger sister and harbors a secret that she fears will destroy her academic future.

Nate is a student on probation who sells prescription drugs to survive, fitting the school's "criminal" stereotype. He lives in a dilapidated house with his alcoholic father and claims his bipolar mother is dead, essentially raising himself. Despite his tough exterior and legal troubles, he is observant, resourceful, and possesses a strong protective streak.

Cooper is Bayview High's star baseball pitcher, heavily scouted by colleges and major league teams. His father has relentlessly groomed him for sports success, moving the family to California specifically for year-round training. Cooper maintains a perfect "golden boy" facade but hides a deeply personal secret that he fears will destroy his family's expectations and his career.

Addy is a popular beauty queen who entirely bases her identity around her controlling boyfriend, Jake. Raised by an image-obsessed mother who teaches her that securing a male provider is her only path in life, Addy acts as a passive sidekick in her own existence. Following Simon's death and the exposure of her secrets, she is forced to figure out who she is without a relationship defining her.

Simon is the notorious creator of "About That," a highly accurate gossip app that terrorizes the student body of Bayview High School. He views himself as the "omniscient narrator" of his classmates' lives, harboring deep resentment over his lack of popularity despite his intelligence. His sudden death from a peanut allergy in detention sets off a massive murder investigation.

Jake is a popular football player at Bayview High and Addy's boyfriend since freshman year. He is highly controlling, dictating how Addy dresses and who she spends time with, demanding total honesty and loyalty. He reacts to stressful situations with cold containment or sudden outbursts of physical violence.

Janae is an alternative, "sorta-Goth" student who serves as Simon's sole loyal companion before his death. In the aftermath of the tragedy, she becomes increasingly anxious, isolated, and physically diminished. She initially lashes out at the four suspects before forming a surprising connection with Addy.

Ashton is Addy's older sister, whose early marriage to a man named Charlie is rapidly disintegrating. Unlike their image-obsessed mother, Ashton encourages Addy to find her own identity outside of male approval. She provides a safe emotional refuge for Addy as the police investigation intensifies.

Maeve is Bronwyn's younger sister and a survivor of childhood leukemia. She is highly intelligent and technologically gifted, using her computer skills to investigate the digital footprint left behind by Simon. She provides Bronwyn with both emotional support and crucial investigative breakthroughs.

Nonny is Cooper's grandmother, a wealthy and supportive figure in his life. She suffers a heart attack during the investigation but remains fiercely loyal to Cooper, gently encouraging him to be true to himself even when his father is unyielding.

Keely is Cooper's beautiful and genuinely sweet girlfriend. Though she fits the stereotypical role of the popular jock's girlfriend perfectly, she feels increasingly invisible in their relationship as Cooper becomes more distant.

Eli is a defense attorney who works for the nonprofit organization Until Proven. He publicly criticizes the local police for their narrow focus on the four student suspects, prompting Bronwyn to relentlessly pursue his legal representation for Nate.

Kris is a German college student who maintains a highly secret relationship with Cooper. He is patient with Cooper's need for privacy but eventually steps up to help the group decode the clues surrounding Simon's death.

TJ is a football player and Jake's close friend. He shares a secret past with Addy that makes him a central figure in the blackmail material Simon intends to publish.

Mikhail Powers is the host of the popular true-crime show *Mikhail Powers Investigates*. He brings national media attention to the "Bayview Four," significantly shifting public opinion by criticizing the police investigation and examining Simon's darker internet history.

Officer Lopez is Nate's probation officer. Unlike many authority figures in Nate's life, she actively tries to help him, encouraging him to attend support groups and warning him about the dangers of his ongoing drug dealing.

Mr. Avery is the strict teacher who assigns detention to Bronwyn, Nate, Cooper, Addy, and Simon. He enforces an anti-technology rule in his classroom and temporarily leaves the room during a staged car crash, leaving the students unattended when Simon collapses.