Arthur is an impoverished English boy born with an extraordinary memory. Dubbed "King Arthur of the Sewers and Slums," he possesses a profound affinity for languages and a deeply sensitive nature. He begins working as an errand boy at a printing press, where a chance encounter with a book ignites his fascination with the ancient civilizations of Mesopotamia. This interest shapes his trajectory toward the British Museum, where he dedicates himself to deciphering cuneiform tablets.

Narin is a nine-year-old Yazidi girl living in modern-day Turkey. She was born with a congenital condition causing her to steadily lose her hearing. Deeply connected to her cultural heritage, she loves listening to her grandmother's stories about their ancestors and the history of Mesopotamia. She is a gentle, observant child who soon faces the terrifying reality of religious persecution when militant forces sweep through her region.

Zaleekhah is a 31-year-old Turkish English hydrologist living in London in 2018. Having lost her parents in a flash flood at age seven, she harbors a complex relationship with water, viewing it with both fear and professional reverence. She struggles with depression and a persistent sense of isolation. Following the collapse of her marriage, she seeks a true sense of home and belonging.

Besma is Narin’s grandmother and a highly respected figure in her Yazidi community. She is a water dowser, a gift inherited from a long line of maternal ancestors who were seers. She serves as the keeper of her family's history, determined to ensure Narin experiences their sacred traditions despite the looming threats in the region.

Uncle Malek is Zaleekhah’s wealthy, successful uncle who took her in after she was orphaned. A member of the House of Lords, he places an intense, almost ruthless premium on family loyalty. While he provided Zaleekhah with a comfortable upbringing, his demanding nature and rigid interpretations of loyalty create friction with those around him.

Nen, short for Brennen, is an Irish woman who owns the houseboat Zaleekhah rents, as well as a tattoo parlor opposite the British Museum. She has a deep fascination with Ancient Mesopotamia and tattoos exclusively in cuneiform script. Having navigated her own struggles with mental health and addiction, she offers Zaleekhah a grounded, empathetic connection.

Leila is a faqra (diviner) and the adopted daughter of a Yazidi sheikh in the 19th century. She possesses the ability to foresee the future, a gift that brings her both reverence and deep sorrow in her community. She is deeply connected to her people's traditions and shares a profound, unspoken bond with Arthur.

Arabella is a "tosher" who scavenges the polluted shoreline of the River Thames to survive in 19th-century London. Trapped in an abusive marriage and struggling with severe melancholy, she finds life overwhelmingly difficult, yet she forms the foundational emotional core of Arthur's early years.

Khaled is Narin’s father, a traveling musician who plays the ancient qanun at weddings and community events. He is an eternal optimist who believes that music and celebration will always endure, a belief that makes him dangerously hesitant to uproot his family despite encroaching violence.

Dr. Birch is the Keeper of Oriental Antiquities at the British Museum. He is an educated, observant man who recognizes Arthur's unprecedented ability to decipher ancient cuneiform tablets, offering the impoverished boy a chance to turn his passion into a legitimate career.

Helen is Zaleekhah's cousin and Uncle Malek's daughter. Growing up in the same household, she and Zaleekhah were exceptionally close as children, but the pressures of adulthood and Helen's intense focus on her sick child have caused a rift between them.

The famous 19th-century novelist. He takes an interest in Arthur's prodigious memory and curiosity, providing the impoverished boy with suitable clothing so he can finally visit the British Museum and pursue his interest in Mesopotamia.

Salma is a Yazidi woman captured alongside Narin. Having known Narin's grandmother, she attempts to protect the young girl by using the family's reputation as seers to frighten their captors.

Ashurbanipal is the "Librarian King" of the Assyrian Empire in the 640s BCE. Famed for his vast collection of clay tablets, including The Epic of Gilgamesh, he is a man of intense intellect juxtaposed with horrifying brutality toward his subjects and enemies.

Mr. Bradbury is a joint partner at the Bradbury & Evans publishing house. He is a kind and observant employer who notices Arthur's potential and gives him access to the press's books, unwittingly sparking the boy's lifelong obsession with ancient history.