Sloane is a 33-year-old nail technician who lives with her mother in a rent-controlled apartment in Carroll Gardens, Brooklyn. Struggling with deep insecurity regarding her working-class background, she frequently fabricates stories about her identity, name, and profession to make herself appear more impressive to strangers. She quickly becomes infatuated with the wealthy Lockhart family after a chance encounter, viewing their lifestyle as an aspirational ideal. Hoping to embed herself in their world, she eagerly accepts a position as their part-time nanny.

Violet is a wealthy 31-year-old wife and mother living in an expensive brownstone in Cobble Hill, Brooklyn. She grew up with a large trust fund in California but moved to New York City to support her husband's career ambitions. Outwardly, she possesses a chic, sophisticated style that commands attention, but she privately struggles with the pressures of maintaining a curated image. She welcomes Sloane into her home, observing the new nanny's behavior and eager attempts at emulation with quiet intensity.

Jay is Violet's husband who is currently launching an online gambling startup. He presents himself as an involved, modern father and an easygoing partner, standing in deliberate contrast to rigid, traditional husbands. He easily draws the attention and admiration of those around him, including Sloane, with his relaxed and flirtatious demeanor. Despite his casual appearance, he frequently neglects his family responsibilities.

Harper is Violet and Jay's five-year-old daughter. She attends the prestigious Mockingbird Montessori preschool and has a mild heart condition that requires careful monitoring. Her minor injury at a local park serves as the catalyst for Sloane's introduction into the Lockhart family's daily life.

Danny is Violet's childhood friend who lived year-round on Block Island. He and Violet share a fiercely loyal bond formed during their teenage years, having spent significant time together at her grandmother's bungalow. He currently works as an EMT and remains deeply protective of Violet.

Laura is an affluent older woman who lives in an Upper East Side penthouse but frequently visits Brooklyn to see her son. Unlike many of the wealthy clients at the spa, she treats the service workers with genuine respect and generosity, making her one of Sloane's favorite customers.

Anne-Marie is a talkative mother from Charlotte, North Carolina, who loves to gossip. She quickly injects herself into the social dynamics of the neighborhood during her summer vacation, eager to learn about the people around her and quick to form opinions based on appearances.

Fitz is Anne-Marie's husband who works in sales. He generally displays a lackluster, unenthusiastic presence compared to his highly energetic and talkative wife.

Allison is a wealthy Brooklyn mother whose children previously attended Mockingbird Montessori. She employed Sloane as a nanny until an unsettling incident involving Sloane wearing her clothes and invading her privacy led to Sloane's termination and a legal restraining order.

Rebecca is Violet's late grandmother, remembered as a free-spirited and accepting woman. She provided Violet with a safe haven from her restrictive parents and eventually left her a substantial eight-figure trust fund, ensuring Violet's financial independence.

Natasha is a nail technician who commutes to Brooklyn from Jersey City. She frequently gossips with Sloane about dating and clients, though their professional relationship is occasionally strained by Sloane's unreliability and frequent breaks.

Chloe is the receptionist at the day spa and a student at Brooklyn College. She maintains a friendly rapport with Sloane and manages the daily flow of affluent clients coming in for appointments.

Nina is the young woman who worked as Harper's nanny before Sloane was hired. Her abrupt departure from the Lockhart household created the vacancy that Sloane eventually fills.