Phoebe is the youngest Oppenheimer sibling, born via surrogate years after the triplets. She grows up in the Brooklyn Heights home primarily with her mother and Lewyn, remaining largely separated from her older siblings' shared history. Feeling as though she missed the defining years of her family's life, she actively asks questions and attempts to piece together the fractured relationships among her relatives.

Lewyn is one of the Oppenheimer triplets. Unlike his siblings, he inherently desires family connection but faces repeated rejection, particularly from his sister Sally. He attends Cornell University, where he hides his identity as an Oppenheimer, explores religion with his Mormon roommate, and eventually develops a deep interest in art history.

Sally is one of the triplets, fiercely independent and eager to distance herself from her family's influence. At Cornell, she gets rid of her possessions and pretends she has no siblings, even though Lewyn lives nearby. She channels her energy into a passion for antique Shaker furniture while privately struggling to accept her sexual orientation.

Harrison is the most reticent and intellectually arrogant of the triplets. Discontented with his progressive education at Walden, he seeks out Roarke, a conservative men's college known for its traditional rigor. He masks his deep insecurities with a sense of superiority and fully immerses himself in right-wing ideology and competitive intellectualism.

Johanna is the matriarch of the Oppenheimer family. Falling in love with Salo early on, she makes it her life's mission to heal his grief and construct a perfect, close-knit family. She goes to great lengths to engineer togetherness among her disparate children, often ignoring the reality of their deep mutual disconnection and her husband's emotional absence.

Salo is the wealthy patriarch of the Oppenheimer family. Traumatized by the tragic accident in his youth, he remains emotionally withdrawn from his wife and children. He channels his buried passion into collecting modern art, specifically Abstract Expressionism, keeping his prized pieces isolated in a Red Hook warehouse away from his domestic life.

Stella is a documentary filmmaker originally from Oakland, California. She is one of the two survivors of the accident that defined Salo's early life. Years later, she focuses her filmmaking on outsider artists like Achilles Rizzoli, a professional pursuit that eventually brings her back into contact with Salo.

Ephraim is Stella's son and a highly accomplished student at Yale. He works on a journalism piece regarding racial passing and maintains a close, remarkably honest relationship with his mother, contrasting sharply with the secrecy of the Oppenheimer siblings.

Rochelle is a sharp, intelligent student from Long Island who becomes Sally's roommate at Cornell. She acts as a caretaker for her hoarding mother and inadvertently gets caught in the middle of the unspoken tensions and secrets between the Oppenheimer siblings.

Eli is a prominent conservative student and author who attends Roarke. Self-taught and highly confident, he quickly becomes a central figure in the campus's ideological divides and offers Harrison a much-desired sense of intellectual belonging and validation.

Harriet is an antique picker who travels to private homes to find valuable collectibles. She lives in an overflowing, run-down house and takes Sally under her wing, teaching her the physical and historical trade of clearing out old estates.

Jonas is an older college student from Idaho who recently completed his Mormon mission. He introduces Lewyn to a worldview entirely different from the Oppenheimers' insular New York upbringing and accepts Lewyn's background without judgment.

Mandy is Salo's girlfriend during his college years. Her tragic death in a car crash fundamentally alters the trajectory of Salo's life and prompts Johanna's initial sympathy and determination to heal him.

Dr. Loring is an academic who introduces Harrison to the conservative Roarke college. His traditionalist views validate Harrison's frustrations with his current school and provide him with an alternative educational path.

Debbie is Johanna's sister. She occasionally visits the Oppenheimer household and acts as a sounding board for Johanna's anxieties about motherhood and family life.