Frannie is an intelligent, observant young Black woman who was born into slavery on a Jamaican sugar plantation called Paradise. Taught to read by her enslaver's wife, she develops a deep love for literature and philosophy. After being brought to London and forced into domestic service, she faces the rigid class structures and deep-seated racism of English society while attempting to assert her own voice and identity.

Meg is the French-born wife of George Benham. Beautiful, charismatic, and eccentric, she rejects the passive roles assigned to women of the English upper class. She suffers from deep melancholy and frequently self-medicates with laudanum, feeling isolated by her husband's dismissive attitude and her status as a cultural outsider in London society.

George is a wealthy London intellectual who views himself as a benevolent scientist and benefactor. In reality, he is a deeply hypocritical and abusive man who demands total submission from his household. He is obsessed with his public reputation and uses pseudoscientific theories to justify his mistreatment of others, treating both servants and his wife as property.

Mrs. Linux is the severe and fiercely racist housekeeper of the Benham household. She rigidly enforces English class hierarchies and deeply resents anyone who threatens the established social order. She directs intense hostility toward Frannie, viewing her presence as an affront to a proper English home, and silently judges Meg for her French origins.

Langton is Frannie's former enslaver, a brutal man who conducts horrific pseudoscientific experiments on his Jamaican plantation. Driven by a desire to prove the inherent superiority of white men, he forces Frannie to assist in his grisly work. When his plantation faces financial ruin, he flees to London, bringing Frannie with him before callously giving her away.

Miss-bella is the bored, chronically ill wife of John Langton. Confined to the Paradise plantation, she views the enslaved people around her primarily as sources of entertainment. She decides to teach Frannie to read purely out of a desire to cure her own boredom, displaying a fickle and dangerous brand of attention.

Phibbah is an enslaved woman on the Paradise plantation who possesses immense knowledge of local botany and medicine. She acts as a fierce protector for young Frannie, teaching her how to survive the dangers of the Langton household. She uses sharp wit and backtalk to manage Miss-bella, understanding exactly how to manipulate her enslaver's boredom to survive.

Pettigrew is the defense attorney tasked with representing Frannie at the Old Bailey. He works to build a plausible defense for her against overwhelming public prejudice, frequently advising her to suppress the more controversial elements of her story in order to save her life.

Pru is a kitchen maid in the Benham household who shares living quarters with Frannie. While she is cautious of angering Mrs. Linux, she occasionally disobeys orders to show Frannie kindness, treating her burns and offering practical warnings about the volatile nature of their employers.

Olaudah, known professionally as the boxer "Laddie Lightning," is a formerly enslaved man brought to England as a child by George Benham. After securing his freedom, he builds a successful career as a prizefighter. He is highly critical of white abolitionists and holds a cynical, realistic view of the limited options available to Black people in London.

Sal is a Black woman working at a London brothel who becomes one of Frannie's closest confidantes. Having bought her own freedom after being brought to London, she is fiercely independent and practical, offering Frannie a grounded perspective on survival outside the servant class.

Hephzibah, often referred to as Hep, is a wealthy woman in Meg's inner circle. She views Frannie with suspicion and disdain, assuming her to be uneducated based entirely on her race. She serves as a representation of the casual, entrenched prejudice of the English upper class.

Jessop is the aggressive prosecuting attorney attempting to secure Frannie's execution at the Old Bailey. He actively smears her character, using racist rhetoric and leaning heavily on the prejudiced accounts of the Benham servants to sway the jury.

Pomfrey is a former enslaver who transitioned to shipping human skulls for scientific study after the slave trade was restricted. He is lewd and utterly dismissive of Frannie, later reappearing to offer her a place at a London brothel when she is destitute.

Mrs. Slap, who goes by Mrs. Austen to her neighbors, is the madam of a London brothel known as the School-House. She expects her workers to cater to the specific, often violent demands of their wealthy clientele, viewing her business through a strictly pragmatic lens.

Casterwick is the butler-valet of the Benham household. He is one of the few servants who shows visible discomfort when Mrs. Linux physically abuses Frannie, though he does not possess the power to stop her.

Charles is a manservant working for the Benham family. He views Frannie as an oddity and a target for ignorant questions, later providing hostile testimony against Meg and Frannie during the murder trial.

Sir Percy is George Benham's brother. Deeply entrenched in the English upper class, he prioritizes his family's reputation above truth and justice, actively intimidating witnesses to ensure no scandal tarnishes the Benham name during Frannie's trial.

Tomkin is the advising lawyer assisting John Pettigrew with Frannie's defense. He helps formulate the strategy to blame Frannie's alleged actions on a laudanum-induced state rather than premeditated intent.

Dr. Fawkes is a physician called upon to treat Meg's severe melancholy. His traditional treatments prove entirely ineffective against her deep-seated depression and isolation.

Pears is a medical professional who assisted in the autopsy of the Benhams. Though he holds crucial medical knowledge regarding the nature of the deaths, he is highly susceptible to intimidation from wealthy English families.

Thomson is an English apothecary who visits Jamaica to gather information on medicinal botany. He completely appropriates Phibbah's extensive knowledge, publishing a book in London that claims her discoveries as his own without offering her any credit.