Jess is a resourceful English woman who arrives in Paris seeking refuge with her half-brother after leaving a waitressing job under troubled circumstances. Having grown up in the foster care system, she is financially insecure and inherently distrustful of authorities. She is highly observant and relies on her finely tuned survival instincts when she realizes her brother is missing from his apartment.

Ben is a charming British journalist who recently relocated to a luxurious Parisian apartment building. He possesses a chameleon-like ability to fit into elite social circles despite his humbler origins. His sudden, unexplained disappearance serves as the central mystery driving his half-sister's investigation.

Nick is a young, wealthy resident of the building who presents himself as an angel investor in tech development. He is an old university friend of Ben's from a decade ago and offers to help translate and interface with the Parisian police for Jess. He lives a luxurious lifestyle but seems uncomfortable discussing the true source of his income.

Sophie is a fifty-year-old woman living in the building's luxurious penthouse. She meticulously maintains her glamorous appearance to satisfy her wealthy husband's expectations. Outwardly cold and pragmatic, she secretly struggles under the pressure of anonymous extortion notes that threaten her heavily curated lifestyle.

Mimi is a young woman living with her roommate Camille on the fourth floor. Though she desires adult independence, she remains emotionally immature and heavily sheltered by her controlling father. She develops an intense infatuation with Ben, watching his apartment closely from her window.

The reclusive, elderly caretaker of the apartment building observes the residents from her small loge. She speaks very little but watches everyone carefully, using surveillance rather than confrontation to maintain order in the building.

Antoine is a volatile resident of the building who struggles with alcohol dependency. He is frequently angry and argumentative, particularly following the recent departure of his wife. He views outsiders with extreme hostility and resents the changes occurring within the building.

Jacques is the wealthy patriarch of the Meunier family and owner of the penthouse. He exerts intense pressure on his family members, demanding traditional gender roles, perfect appearances from his wife, and subservience from his children.

Theo is a newspaper editor for the Guardian based in Paris who works with freelance journalists. As a fellow Brit, he provides Jess with her only reliable connection outside the oppressive atmosphere of the apartment building.

Irina is a beautiful dancer working in Paris who is secretly searching for Ben. She approaches Jess hoping for information but flees quickly when she learns Ben might be in danger.

Camille is Mimi's cool, sexually liberated, and attractive roommate on the fourth floor. She moves through the Parisian social scene with an ease that both intimidates and impresses Mimi.

Dominique is Antoine's estranged wife who abruptly leaves the apartment building early in the story. Her departure involves a loud, public argument in the courtyard that awakens the other residents and sets a tense tone for the building.