Friedrich is a twelve-year-old aspiring conductor living in 1933 Germany. He works as an apprentice at a local harmonica factory rather than attending school to avoid the intense bullying he faces due to a large birthmark on his face. He finds solace in a mysterious harmonica he discovers at work.

Mike is an eleven-year-old orphaned piano prodigy living in rural Pennsylvania. Exceptionally tall for his age, he appears older and uses this to protect his brother. He hopes to join a traveling harmonica troupe to secure a better life for them both.

Ivy is an eleven-year-old Mexican-American girl moving to Orange County, California. She is a talented musician forced to attend a segregated school. She balances her parents' practical expectations with her own musical aspirations and her desire to be a "good soldier" while her brother is away.

Otto is a young boy lost in the woods. He reads a story about three cursed princesses and soon encounters them in reality. They entrust him with a special task involving a woodwind instrument.

Three princesses raised by a witch. They are bound by a curse to remain in a woodwind instrument until they can save a life. They share beautiful voices and rely heavily on one another for support.

Martin is Friedrich and Elisabeth's father, a former cellist who gave up his career to raise his children. He quietly opposes the rising intolerance in his country and attempts to maintain relationships with targeted friends, putting himself at risk.

Elisabeth is Friedrich's eighteen-year-old older sister. Initially planning to stay home, she reveals she requested reassignment to a hospital in Berlin to continue her involvement with political youth groups.

Gunter is Friedrich's uncle and Martin's brother. He teaches Friedrich how to play the harmonica and serves as a protective figure for his nephew at the factory.

Frankie is Mike's seven-year-old brother. He is athletic and talkative, heavily relying on his older brother for emotional support following the loss of their mother and grandmother.

Pennyweather is the harsh headmistress of Bishop's Home for Friendless and Destitute Children. She uses the boys in her care for manual labor on local farms and confiscates their personal belongings.

Eunice is a wealthy, grieving woman who agrees to adopt a child only to fulfill the conditions of her late father's will. She is initially cold and resistant to connecting with the boys.

Mr. Howard is Eunice's friend and representative. He arranges the adoption of the Flannery boys, believing that bringing children and music into Eunice's home will help heal her emotional pain.

The Potters are the groundskeeper and housekeeper for the Sturbridge estate. They welcome Mike and Frankie with warmth and try to explain Eunice's difficult behavior.

Fernando is Ivy's older brother who enlists in the army. Before leaving, he asks Ivy to be a "good soldier" at home and encourages her musical talent, sending her money for a concert.

Ivy's parents are hardworking Mexican-Americans who relocate for a supervisory role on a farm. They value practical security over Ivy's musical ambitions but advocate fiercely for her when she faces discrimination.

Susan is a neighborhood girl who befriends Ivy. She lives an isolated life because her father fears losing another child, and she passes on his suspicions about the local Japanese families.

Anselm is a new employee at the harmonica factory. He continuously pressures Friedrich to attend political youth meetings, acting as an informant who poses a threat to the Schmidt family.

Susan's father is a grieving man who lost a son at Pearl Harbor. His trauma makes him suspicious and hostile toward the Japanese-American families in the area.

Kenneth is a young U.S. Marine whose family owns the farm where the Lopez family works. He returns to inspect the property and connects with Ivy over their shared love of music.