Cassie is a 26-year-old detective for the Dublin Murder Squad, currently working a desk job in the domestic violence unit. Having lost both of her parents in a car accident at age five, she often struggles with a sense of isolation. She possesses a natural talent for undercover work but carries psychological scars from her previous assignment. When confronted with a murder victim who perfectly resembles her, she agrees to inhabit the deceased woman's life to catch the killer.

Frank is the seasoned, hard-nosed detective leading the Operation Mirror investigation. He previously worked with Cassie to create the original "Lexie Madison" undercover persona. Currently separated from his wife, he focuses intensely on his work and operates with a highly pragmatic, sometimes manipulative approach to solving cases.

Daniel is a post-graduate student who recently inherited Whitethorn House from his great uncle. He serves as the unofficial leader of his insular circle of friends, fostering a communal, anti-capitalist lifestyle. Having lost his parents, he is deeply devoted to his chosen family and fiercely enforces a rule that the housemates must not discuss their individual pasts.

This young woman is a murder victim who bore an uncanny physical resemblance to Detective Cassie Maddox. Prior to her death, she integrated herself into a group of post-graduate students at Whitethorn House using an alias previously invented by the police. She was pregnant at the time of her death and possessed a secretive nature, leaving behind a coded journal and a trail of unresolved questions regarding her true origins.

Rob is Cassie's former detective partner and closest friend. Their relationship disintegrated following the traumatic events of a previous case, Operation Vestal. Though physically absent from the current investigation, his memory heavily influences Cassie's psychological state and her approach to forming connections with the suspects at Whitethorn.

Sam is a detective on the Dublin Murder Squad and Cassie's current romantic partner. He is highly protective of Cassie and vocal about his reservations regarding Operation Mirror. As the outside investigator for the case, he handles the local inquiries while Cassie operates inside the house, often growing frustrated with the lack of progress and the danger she faces.

Abigail, known as Abby, is an English post-graduate student from Dublin. Unlike her male housemates, she does not come from a wealthy background. She takes on many domestic roles within the house, such as cooking and restoring Victorian dolls, and demonstrates a deep emotional investment in the group's cohesion and well-being.

Rafe is a handsome trust-fund student attending Trinity College. He relies on his family's wealth but rejects his father's traditional expectations, choosing instead to live in Daniel's secluded commune. He frequently utilizes alcohol to cope with stress and exhibits a volatile temper when the group's unity is threatened.

Justin is an English post-graduate student whose father is a lawyer. He struggles with severe anxiety and physical tremors in the aftermath of Lexie's stabbing. Having experienced rejection from his family, he relies heavily on the Whitethorn housemates for a sense of belonging and struggles to manage his immense feelings of panic.

John is a local resident of Glenskehy who holds a deep-seated grudge against Whitethorn House and its inhabitants. He resides with his parents and shares the town's historical resentment toward the March family. He becomes a primary suspect for the local detectives due to his history of vandalism and animosity toward the students.

Ned, also known as Slow Eddie, is Daniel's cousin. He wears a distinctive leather bomber jacket and holds a significant grievance over being passed over for the inheritance of Whitethorn House. He actively seeks to acquire the property to develop it into a profitable commercial venture, such as a spa or luxury apartments.

Simon is Daniel's great uncle, a historical figure in the March family tree. His death triggered a complicated probate process and ultimately resulted in Daniel inheriting the sprawling, decaying Whitethorn House, setting the stage for the students' communal experiment.

O'Kelly is the superintendent of the Dublin Murder Squad. He oversees the division's operations and maintains a tense, somewhat adversarial relationship with the local coroner.

Cooper is the coroner attached to the murder investigation. He delivers the critical medical findings regarding the victim's cause of death and physical condition, providing the detectives with the foundational timeline of the crime.