Nan is an eleven-year-old orphaned girl working as a "climbing child" in Victorian London. Raised initially by a gentle chimney sweep, she now endures a harsh life as an indentured servant under Wilkie Crudd. Despite constant hunger and dangerous working conditions, she retains a sharp mind, an independent streak, and a deeply ingrained instinct to protect younger sweeps.

The Sweep is a kind, inventive man who served as Nan's adoptive father until his mysterious disappearance when she was six years old. He possesses a talent for finding magic in everyday objects, using storytelling to distract Nan from their grinding poverty. He leaves behind his hat and a strange piece of warm char to look after her in his absence.

Charlie begins as a small, sentient lump of coal but quickly grows into a hulking soot golem. Created to be Nan's protector, he possesses an intimidating, monstrous physical form that contrasts sharply with his gentle, inquisitive, and innocent personality. He learns about the world rapidly through Nan's teachings while living in the abandoned House of One Hundred Chimneys.

Toby is an enterprising Jewish boy who makes his living as a mudlark on the banks of the Thames. He scavenges discarded items to repair and sell, keeping his wares in an old soot bag he calls his "emporium." Having known Nan for years, he frequently attempts to assist her, offering practical help, engineering skills, and a steadfast emotional connection.

Wilkie Crudd is the ruthless, impeccably dressed owner of the chimney sweeping business where Nan is indentured. He exploits the desperate children in his employ, prioritizing profits over their safety and basic needs. He is a calculating master who relies on intimidation and physical threats to maintain absolute control over his climbing boys.

Miss Bloom is an educator who grew up in an Orthodox Jewish household before moving to the seminary. She possesses a keen interest in literature and a strong sense of social justice. Upon discovering that Nan can read, she begins lending her books and provides crucial information regarding the mythological origins of golems.

Roger is an older climbing boy working under Wilkie Crudd. Having been sold into the trade by his own parents, he directs his bitter resentment toward his peers, actively making their harsh lives more difficult. He views Nan as his primary rival and frequently attempts to sabotage or physically harm her to secure better treatment from Crudd.

Newt is the newest addition to Crudd's crew of climbing boys. Coming from a formerly comfortable background before being orphaned or abandoned, he is terrified of the dark, narrow flues and cries himself to sleep at night. He looks up to Nan for comfort and guidance in an otherwise bleak existence.

Mrs. Trundle is the cruel housekeeper for Wilkie Crudd. She manages the domestic affairs of The Clean Sweep, enforcing rules through physical discipline and intentionally withholding food as punishment for minor infractions.

Wittles is one of the young climbing boys indentured to Wilkie Crudd. He endures the same harsh conditions as Nan and Newt, working long hours in the dangerous chimneys of Victorian London.

Shilling-Tom is another climbing boy in Crudd's crew. Like the others, he sleeps in the coal cellar and struggles to survive the daily hazards of chimney sweeping, holding on to small treasures to maintain hope.

Prospero is a rat who lives with Toby. He frequently travels with the young mudlark and becomes an early companion to Charlie when the golem first starts exploring his new home.

Dent is a small baby robin that takes up residence in the attic of the House of One Hundred Chimneys. With a malformed wing that prevents flight, the bird becomes a central focus of Charlie's affection and care.