Belle is a Princeton-educated art historian who assumes a false identity to work as the personal librarian for steel magnate J.P. Morgan. Following her parents' separation, she claims Portuguese descent to mask her Black heritage, securing her family's financial stability. She utilizes bold fashion choices and extensive art knowledge to assert authority in the male-dominated field of rare book collecting.

A wealthy financier and industrialist, J.P. dominates the Gilded Age economic structure. He purchases rare art and books to outshine his social rivals and assert his dominance over the New York elite. He expects complete deference from his employees and views his extensive collection as a private monument to his own power.

Genevieve is a white-presenting woman who descends from the elite Fleet family of Washington, D.C. Driven by the trauma of racial violence early in her marriage, she orchestrates her family's transition to living as white New Yorkers. She strictly enforces respectability and modesty, believing these traits will protect her children from scrutiny.

Bernard is an erudite art historian and critic whose writings inspired Belle during her childhood. He moves in elite Boston circles but maintains the aura of an outsider due to rumors about his background and personal life. He publicly advocates for making art accessible to the working class while working in the competitive business of art dealing.

Richard is a prominent civil rights activist and the first Black man to graduate from Harvard. Formerly a philosophy professor at an integrated university, he possesses deep convictions and believes in confronting racial inequality directly. He refuses to participate in his wife's plan to assume a white identity.

Anne is J.P. Morgan's daughter and a prominent figure in New York society. She acts as a fierce protector of her father's legacy and views his new librarian with intense suspicion. She manages the social constraints of her era by keeping her intimate relationships with other women insulated by her immense wealth.

Mary is a writer and lecturer who is married to Bernard Berenson. She lives in an unconventional open marriage that allows both her and her husband the freedom to pursue other romantic connections. She is pragmatic about her husband's nature and maintains a professional partnership with him.

Jack is J.P. Morgan's married son who stands to inherit his father's business and cultural legacy. He recognizes the prestige the Morgan collection brings to his family but approaches the management of this massive wealth pragmatically.

Junius is J.P. Morgan's nephew who recognizes Belle's talent during her time working at Princeton University. His recommendation serves as the catalyst for her entry into the high-stakes world of elite New York book collecting.

Russell is Belle's brother who shares her physical resemblance. He participates in the family's passing strategy and occasionally accompanies Belle to high-profile social events to secure employment opportunities.

Teddy is the youngest child in the Greener family. Because of his age, he has no memory of the family's life before they assumed their white identities and remains entirely unaware of their true racial heritage.

Katrina is a former college classmate of Belle's who has rejected respectable career paths like teaching in favor of political activism. She lives independently in a women's space and actively campaigns for women's voting rights.

Evelyn is another of Belle's college friends who pursues a non-traditional lifestyle. She works as an artist and spends her time among the young bohemians of Greenwich Village, fully embracing the independence of the era.

Grandma Fleet is the matriarch of Genevieve's side of the family. She represents the elite Black district of Washington, D.C., and strongly disapproves of Genevieve's decision to sever ties with their heritage to pass as white.

Mozart is Belle's uncle who remains in Washington, D.C. He recognizes the extreme danger of his sister's choice to pass as white and explicitly warns Belle about the risks of being unmasked by powerful people.

Elsie is a prominent woman in New York society and a close associate of Anne Morgan. She presents herself as a feminist but participates in the cutthroat gossip of the city's elite, specifically probing into rumors about Belle's ancestry.

Bessie is a woman who shares a close, intimate relationship with Anne Morgan. She travels internationally with Anne, benefiting from the social insulation that their wealth provides for their unconventional lifestyle.