Rosanne is a sixteen-year-old orphaned girl living with synesthesia, a neurological condition that causes her to perceive sounds as vivid colors and shapes. After her parents and younger brother die in a car accident, she works as a live-in maid and assistant cook at the Rousseau Vineyard. She drops out of school to avoid sensory overload but reads constantly. Following her family's death, she actively searches for a sense of belonging, which leaves her vulnerable to exploitation by her employers.

Helen is Truman Calvert's sister, an independent woman who spends four decades working as a nanny in cities like London, Paris, and Vienna. She refuses conventional career paths, choosing instead to travel and form strong bonds with the families she serves. In Austria, she closely monitors the state's new health laws and actively plots to protect the children in her care. She carries a deep sense of moral duty that drives her actions both abroad and upon her return to California.

Truman is the co-caretaker of the Rousseau Vineyard and Rosanne's employer following the death of her family. He walks with a slight limp from a World War I injury and typically defers to his wife regarding business and household management. He sits quietly in the parlor drinking whiskey and listening to music his wife dislikes. Despite his steady outward appearance, he betrays his ward's trust when left alone with her and hides significant assets from his spouse.

Celine runs the Rousseau Vineyard alongside her husband and assumes guardianship of Rosanne. She treats the orphaned girl strictly as hired help, setting firm household rules and distancing herself emotionally. Harboring unresolved grief over a past miscarriage, she maintains tight control over her home. She takes decisive, unforgiving action when she discovers a disruption in her household structure.

Amaryllis is Rosanne's infant daughter, born while her mother is confined to a psychiatric institution. Named after the winter-blooming flower gifted by Helen Calvert, she represents a source of deep joy and purpose for her mother in an otherwise grim environment. She is quickly placed into the foster system, leaving her isolated early in life.

Wilson is the only living child of Truman and Celine Calvert. He is a college student who occasionally returns home for short holidays. Friendly and observant, he remembers childhood conversations with Rosanne, including her early attempts to explain her visual perceptions of sound.

Brigitta is the youngest daughter of the Maier family in Vienna. She lives with significant physical disabilities and developmental delays, requiring constant assistance. She is a cheerful child who brings immense joy to her family, though her condition places her in severe danger under the newly enforced laws of the state.

Johannes is an officer in the reformed Austrian military and Brigitta's father. He attempts to balance his demanding career with his family responsibilities during a highly volatile political period. He frequently dismisses his wife's concerns regarding the state's new health policies, choosing to trust his military superiors instead of acknowledging the danger to his child.

Martine is Brigitta's mother and a resident of Vienna. She pays close attention to local gossip regarding the government's treatment of children with disabilities. Her growing anxiety over her youngest daughter's safety drives her to consider desperate measures to protect her family from the health inspectors.

Belle is a young woman committed to the Sonoma institution under accusations of inappropriate behavior. Resourceful and openly defiant, she quickly befriends Rosanne and begins plotting a way for them to escape the facility. She uses her charm to manipulate the hospital staff and gather critical information.

Dr. Townsend is a physician and therapist at the Sonoma State Home for the Infirm. He evaluates the female patients and takes a particular interest in Rosanne's synesthesia. He employs harsh, punitive methods, using small electrical shocks in an attempt to cure his patients of their visual perceptions.

Mrs. Clark is the older woman who manages the halfway home in Petaluma where Rosanne is transferred at age nineteen. Unlike the institutional matrons, she treats the young women with genuine warmth, providing a safe and structured environment for them to learn independence and find local employment.

Mrs. Grissom is a social worker with County Human Services. She is a practical government employee tasked with moving wards of the state between various homes and institutions, showing little emotional investment in the vulnerable individuals she transports.

Charlotte is a young patient at the Sonoma institution who briefly connects with Rosanne. Her sudden disappearance following a mandatory surgical procedure serves as a grim warning to the other residents about the facility's true nature.

Ruth is a resident at the psychiatric institution. Her medical chart, which Rosanne secretly reads, provides the first concrete evidence of the irreversible surgical procedures being performed on the female patients without their consent.

Rudy is an orderly at the psychiatric hospital. He becomes infatuated with Belle, who manipulates his feelings to secure keys and information necessary for an escape attempt.

Stuart is Dr. Townsend's son, holding a part-time training position at the Sonoma facility. He frequently monitors the patients and catches Belle mocking him, leading to increased friction between the staff and the residents.

Mrs. Crockett is the institution matron at the Sonoma State Home. She represents the cold, bureaucratic enforcement of the hospital's rules and endures a sharp parting rebuke from Rosanne regarding the facility's routine abuses.

Lila is a local friend of Helen Calvert. When Helen returns to California, Lila and her husband eagerly open their home to her, providing crucial local knowledge and emotional support during Helen's search for her family.

George is Lila's husband and a loyal friend to Helen. He supports his wife in offering Helen a permanent place to stay and assists in gathering vital information about the local institutions.

Emilie is Brigitta's schoolmaster in Vienna. She uncovers the truth about the government's systematic health policies and bravely organizes an underground effort to transport targeted children across the border to safety.

Wilhelm is a young deaf boy living in Austria who is targeted by the state's health inspectors. Helen escorts him on a dangerous train journey to seek refuge in Switzerland under the guise of visiting a relative.

Franz is a Swiss border officer who interrogates Helen during her attempt to smuggle Wilhelm. Though he quickly deduces her true intentions, he proves to be sympathetic to the children's plight and offers quiet assistance.

Mrs. Sommers is the director of the Fairbrook Children's Home in Oakland. She manages the facility's records and assists Helen by providing crucial information about the child she seeks.