Stephen is an ambitious working-class detective currently assigned to the cold case squad. He views the new evidence brought by Holly as an opportunity to transfer to the active murder squad. Because of his own humble background, he observes the wealthy students of St. Kilda's and Colm's with an objective detachment rather than resentment. He possesses strong interpersonal intelligence and quickly establishes a rapport with the teenage witnesses.

Antoinette is the lead investigator on the Dublin murder squad. Working in a male-dominated department, she has developed a tough, abrasive exterior to fend off frequent harassment and microaggressions from her colleagues. She approaches cases methodically and is a sharp judge of human behavior, easily seeing through the polished facades of the boarding school students.

Holly is a sixteen-year-old student at St. Kilda's Catholic boarding school. Having a detective for a father, she possesses a sophisticated understanding of police procedure and evidence handling. She is highly protective of her close-knit friend group and openly scorns the conventional social standards that dictate behavior among her peers.

Selena is a thoughtful, middle-class student who stands out among the affluent population of St. Kilda's. Raised partly on a commune by her father, she holds a dreamy, spiritual worldview that rejects traditional religious dogma. She values genuine connections but finds herself at the center of damaging rumors regarding the murdered boy from the neighboring school.

Julia is an assertive and highly analytical member of Holly's clique. Following a frightening encounter with a boy named James, she convinces her friends to swear off romantic entanglements entirely. She is exceptionally cool under pressure, routinely sparring verbally with rival students and maintaining a steady composure during police interviews.

Becca is a wealthy but socially isolated student whose parents live in Dubai. She entirely disregards the rigid fashion and beauty standards enforced by the popular girls, choosing to wear jeans and avoiding makeup. This nonconformity makes her a frequent target for mockery, but she relies heavily on the profound, almost mystical bond she shares with her three best friends.

Chris is the affluent and popular teenage boy whose murder on the grounds of St. Kilda's drives the novel's investigation. In life, he utilizes his wealth to maintain multiple secret cell phones, allowing him to conduct hidden relationships with several girls simultaneously. Despite his manipulative dating habits, he occasionally demonstrates genuine compassion for social outcasts.

Joanne is the ruthless, unofficial leader of St. Kilda's popular clique, whom their rivals dub the "Daleks." She strictly enforces conformity among her friends, requiring heavy makeup, straightened hair, and specific clothing brands. Deeply invested in maintaining her high social status, she uses cruelty and blackmail to control those around her.

Gemma is a confident and affluent student operating as Joanne's second-in-command. Having a father who is a solicitor gives her a strong sense of legal protection, allowing her to act more autonomously than the rest of her clique. She openly flirts with Stephen during her interview but remains tight-lipped about the group's secrets.

Orla is an insecure student residing at the bottom of Joanne's social hierarchy. Desperate for approval, she willingly accepts Joanne's public mockery in exchange for the protection of belonging to the popular crowd. The detectives identify her as the weak link in the group and exploit her anxiety during interviews.

Alison is a highly anxious and skittish member of the popular crowd. She lacks the cunning of Joanne and Gemma, often failing to recognize when she is being manipulated. Prone to melodramatic reactions, she claims to have seen the ghost of the murdered boy haunting the school corridors.

Frank is an experienced Dublin detective and Holly's protective father. He understands the internal politics of the police department and attempts to use his influence to shield his daughter from intense scrutiny, actively warning Stephen to distance himself from Antoinette Conway.

Aileen McKenna is the strict headmistress of St. Kilda's. She established the "Secret Place" bulletin board under the guise of offering students a safe outlet for expression, though she pragmatically uses it as a surveillance tool to monitor the girls' hidden conflicts.

James is a student at Colm's boys' school. When he attempts to sexually assault Julia and she forcefully rejects him, he retaliates by spreading malicious rumors about her reputation.

Finn is a student at Colm's who possesses technical skills, specifically rewiring the timed locks on the school doors to sneak out at night. He challenges Julia to meet him on the grounds after curfew.