Alouette is an 18-year-old woman living in 1664 Paris. Although she possesses a brilliant mind for chemistry and color, the strict laws of the dyers' guild forbid women from practicing the craft, forcing her into grueling labor as a washerwoman. She secretly studies the polluted Bièvre river and cultivates a hidden garden of dye plants to conduct her own unsanctioned experiments. Her ambition drives her to create a revolutionary blue color using dangerous materials.

Kristof is a Dutch psychiatric resident doctor working at Hôpital Sainte-Anne. He carries immense grief over the loss of his sister and adopts a highly compassionate, individualized approach to treating traumatized soldiers. As the threat of German occupation grows, he finds distraction by exploring the ancient limestone quarries hidden beneath the city streets. He is lonely but finds a sense of warmth when he befriends the Jewish family living in the apartment below his.

Sasha is a 12-year-old Jewish refugee from Warsaw living in Paris with her parents and younger brother. Highly intellectual and precocious, she challenges herself to memorize ancient texts using a mental mapping technique. As the city becomes increasingly hostile, she relies on her academic pursuits and stories of transformation to shield herself from the frightening reality outside her door.

Étienne is a young, stoic quarryman working in the dangerous subterranean mines of 17th-century Paris. After a cave-in killed his father, he took on the responsibility of providing for his grandmother and his deceased cousin's two children. He views his grueling labor as an unavoidable fate until a chance encounter in the village alters his perspective.

Alesander is a charismatic Basque architecture student living in Paris during the onset of World War II. He spends his time mapping the vast, abandoned limestone quarries and ossuaries hidden beneath the city streets. He is fascinated by the history carved into the tunnel walls and uses his knowledge of the subterranean passages to guide his friends through the dark.

Claude is the wealthy and powerful head of the Gobelin family, which maintains an absolute monopoly over the dyers' guild in Paris. He strictly enforces the laws dictating who is allowed to practice the craft and what colors can be produced. He uses his influence to crush any independent artisans who threaten his family's financial and social dominance.

Dr. Claudel is the newly appointed superior at the psychiatric hospital, tasked with preparing the facility for wartime casualties. He is an ambitious administrator who champions experimental and violent treatments, such as electroshock therapy, prioritizing rapid, clinical efficiency over patient well-being. He views the men under his care as logistical problems to be solved for the state.

Monsieur Moreau is a cold physician at the notorious Salpêtrière hospital, an institution used to confine women deemed troublesome by society. He strictly adheres to the era's dominant humoral theory, believing that health depends on balancing bodily fluids. He subjects the women in his care to excruciating treatments, convinced that breaking their will is a necessary medical correction.

Henriette is a skilled herbalist who suffered a severe mental health crisis following a series of stillbirths. She abandoned her family when her daughter was only six years old, moving to a nearby town to survive on her own. Her sudden reappearance in Saint-Marcel causes significant turmoil for the family she left behind.

René is a master dyer obsessed with perfecting a signature red color, known as "Voland's Scarlet." He is a talented artisan but operates under the strict, oppressive oversight of the powerful dyers' guild. He attempts to protect his daughter by forbidding her from experimenting, though he secretly appropriates her brilliant discoveries for his own legacy.

Felix is a Polish Jewish refugee who fled Warsaw with his family to build a new life in Paris. He works as a skilled watchmaker and tries to maintain a sense of calm and structure in his household. He frequently plays the Japanese strategy game Go, an object he carried with him across Europe as a symbol of endurance.

Rachel is a pragmatic and resilient mother managing a household in an increasingly hostile 1940s Paris. Pregnant with her third child, she uses intense resourcefulness to stretch meager rations and mend clothing. She maintains an unnerving calm during crises, focusing entirely on the immediate physical needs of her family.

Julian is a psychiatric resident doctor working alongside Kristof at Hôpital Sainte-Anne. He is deeply disturbed by the violent treatments advocated by the hospital administration but feels paralyzed by his responsibilities at home. He represents the painful compromises made by ordinary individuals attempting to survive under a shifting, dangerous regime.

Sylvine is a stoic, hardened inmate at the Salpêtrière hospital. Having been imprisoned for adultery, she is deeply familiar with the brutal realities and corrupt guards within the institution. She uses her knowledge to help other women navigate the daily abuses, distributing contraband herbs and orchestrating small acts of defiance.

Marguerite is a quiet, observant woman confined within the walls of Salpêtrière. While she rarely speaks out, she secretly maintains a small ledger documenting the various abuses and crimes committed by the hospital's staff. Her quiet resistance makes her an essential ally in a place designed to erase women entirely.

Madame Gagnon is the concierge of the Parisian apartment building housing Kristof and the Brodsky family. She is a bitter, opportunistic woman who openly embraces the rising antisemitism in the city, using the shifting political climate to harass and threaten the vulnerable residents in her building.

Gérard is a Jewish teenager navigating the increasingly dangerous streets of occupied Paris. He plays the violin—specifically focusing on difficult pieces by Bach—to focus his mind and cope with the stress of segregation. He forms a fast connection with a classmate over their shared interest in classical literature.

Ursula is an Austrian nurse working at Hôpital Sainte-Anne. Underneath her professional exterior, she is highly aware of the political dangers infiltrating the medical field. She serves as a quiet moral compass in the hospital, undertaking small, dangerous acts of subversion against the new administration.

Christiane is a frail, orphaned girl who has been raised entirely within the walls of the Salpêtrière hospital. Starved for kindness, she eagerly takes to learning about medicinal plants, declaring herself an apprentice to the older women working in the dirt.

Lucienne is a senior dyer working in the Saint-Marcel factory. She is a pragmatic woman who has learned to survive by keeping her head down and accepting the rigid laws of the guild, frequently advising her younger coworkers that spite is a useless emotion.

Mémé is an elderly woman living in a cramped Parisian home. Following the death of her son in a mine collapse, she relies on her grandson to provide for the family. She offers grounded, unsentimental advice to keep the household functioning.

Thomas is a young boy whose mother died, leaving him in the care of his older cousin. He struggles to adjust to life in the city, representing the fragile innocence that his guardian risks his life daily in the quarries to protect.

Marie is a young orphaned girl living in 17th-century Paris. Along with her brother, she forms the core reason her guardian continues to endure the life-threatening labor of the underground mines.

Rald is the young son of Polish Jewish refugees living in Paris. Too young to fully comprehend the political forces tightening around his family, he relies completely on his parents and sister to interpret the frightening changes occurring in their building and city.

Monsieur Mercier is a French gendarme who benefits directly from the persecution of his neighbors. He loudly plays pro-Vichy radio broadcasts and snoops around the building, acting as the immediate, domestic face of state collaboration.