Sophie is a 12-year-old girl with unruly, lightning-colored hair and pale skin. Rescued from a maritime disaster at the age of one, she lives in Victorian London with her eccentric guardian. She stubbornly refuses to conform to traditional expectations, preferring to wear boys' trousers, and holds an unshakable belief that her mother survived the wreck. Drawn to high places and the sound of the cello, she is fiercely determined, imaginative, and remarkably agile.

Charles is a 36-year-old unmarried scholar who lives in a cluttered, book-filled London mansion. He prefers literature and poetry over rigid social conventions, speaking to cats in French and birds in Latin. As an eccentric but profoundly devoted guardian, he raises his ward without strict schedules or traditional Victorian schooling, believing that love is the most important qualification for parenthood. He is tall, strong, and deeply supportive of unconventional thinking.

Miss Eliot is a gray-haired functionary for the National Childcare Agency in London. She focuses on strict conformity, concerning herself deeply with propriety, gender roles, and neatness. She finds Charles's unstructured parenting style appalling and vehemently objects to a young girl wearing boys' clothing. Her voice is sharp, and she relies on strict rules to maintain control over the children in her jurisdiction.

Matteo is an unhoused Parisian boy who lives entirely above the city streets, making his home on the slate roof of the law courts. He survives by hunting pigeons with a bow and arrow and foraging from high windowsills. Having spent a traumatizing period in an orphanage after a violent street fight, he refuses to set foot on the ground or enter enclosed spaces. He is wary, immensely agile, and fiercely protective of his elevated territory.

Vivienne is a French cellist known for her exceptional talent and unconventional style. She possesses a lawless-looking mouth, short fair hair, and a tendency to play solemn music at a wildly fast, double-time pace. Unconcerned with societal expectations for women, she freely wears men's clothing and takes her music to the rooftops of Paris. Her mysterious disappearance after a shipwreck leaves behind only a legacy of music and defiance.

Anastasia is a 13-year-old tree-dwelling girl in Paris. She and her sister survive by sleeping in hammocks strung high in the city's tallest trees, far above the notice of ordinary pedestrians. She is blonde, fiercely independent, and highly articulate, having learned English by eavesdropping on tourists in the parks below. She adheres to a strict moral code, always paying for whatever items she must take to survive.

Safi is a 10-year-old girl who lives among the treetops of Paris with her older sibling. She has a hardened, severe expression that suggests she has survived significant hardship despite her young age. She rarely speaks when others are around, preferring to observe quietly and let her physical actions do the talking. Despite her small stature, she is an exceptionally fierce and capable fighter when threatened.

Gérard is a tall, long-legged boy who makes his home in the lofty architecture of the Notre Dame cathedral. He possesses extraordinary hearing, capable of picking up the faint sounds of a harmonica from halfway down the river. He requires payment for his services so he can purchase votive candles for the cathedral. Beyond his combat skills, he has a beautiful singing voice that commands attention.

Martin is an inspector for the National Childcare Agency in London. He works directly alongside his sister to enforce the city's strict standards regarding childhood education, diet, and decorum. He lacks imagination and views any deviation from standard Victorian practices as an unacceptable threat to a child's welfare.

Mr. Lille is a hard-eyed shop assistant with a sneering mouth and extremely conservative views on music and gender. He heavily disapproves of women playing the cello, viewing it as unladylike. He holds a long-standing grudge against a specific former customer whose shabby clothing and fast-paced playing style deeply offended his rigid sensibilities.

The Chief Police Commissioner is a high-ranking law enforcement official with a rigid face and a deeply corrupt agenda. He uses his considerable power to suppress information and intimidate private citizens who ask too many questions. Operating out of an imposing building filled with locked archives, he actively covers up a massive maritime insurance scam to protect crooked shipowners and complicit officials.