Hong Jaein

She possesses absolute pitch and excels in sight-singing and ear training, with ears so sensitive and accurate she can tell which pianist is playing after hearing just one phrase. There was only one genius who caught her ear: Bio. Since then, she has habitually made a fuss about marrying him. After becoming an adult, she quit the viola and got a job as Bio's manager, still following him around and liking him.

Choi Jungyo

Forced by a school teacher, he happens to visit a concert hall and watches a performance by Bio, whom he met only once at a competition as a child. Immersed in subtle emotions while recalling old memories, Jungyo steps onto the stage—a place he never once coveted. This impulsive action becomes the catalyst that leads him to an unexpected fate. Eventually, he transfers to the first-year music class of Korea Arts High School, where only geniuses gather. People assume he must have donated grass for the school field or a couple of Steinway pianos, but looking at his appearance, such assumptions don't fit at all; he strangely gives off the vibe of a delinquent youth. Into such a life, an unforgettable, fateful friendship approaches. While living that life, he is kidnapped and shot on his way to a competition. His current status is unknown; no one knows his whereabouts, and he has never appeared before Jaein or Bio again.

Kang Jihye

A former ballerina with an elegant and dignified appearance. She exudes nobility and is unmatched in manners and culture, but her eyes hold a coldness and chill typical of sensitive artists that makes her difficult to approach. The daughter of a prominent musical family, she graduated from Korea Arts High School and joined a famous ballet company in Germany, only to shock the world by returning as a single mother. No matter how much she was asked who the father was, she never answered and chose the life of a single mother. She is Bio's mother. Upon realizing Bio possessed immense talent, she transformed into a fierce 'stage mom.' Jungyo was kidnapped on his way to the competition, and circumstances suggest Jihye was behind it.

Han Hyeonjeong

Jaein's mother and the CEO of a classical artist management agency. She married a man from a chaebol family after a passionate romance, but he was a serial philanderer. She eventually divorced him and successfully started her own agency. Jaein, who takes after her, is Hyeonjeong's only hope, but her immature daughter keeps making a fuss about marrying Bio. While she respects Jihye and Bio as artists, she finds them personally very difficult to like.

Do Hyunsoo

A handsome man with long fingers, a deep voice, and a relaxed way of speaking. A graduate of Korea Arts High School, he studied in the US and taught there before returning to Korea as a university professor. He advises Bio that music is not just technique, but sincerity and story rising from deep within, and that he should put human anguish into his music. One day, he spots Jungyo and immediately recognizes him as a rough diamond. As a true mentor, Hyunsoo helps the impoverished Jungyo by letting him use his practice room and giving him free lessons. He hasn't seen Jungyo since he went missing during the competition.

Kang Seungwoon

A genius musician born around the time of the Korean War who was called a prodigy from the age of seven. He is a taciturn, self-righteous, and controlling man. Due to the nature of his profession as a conductor and having always been at the top, combined with his musical perfectionism, his dogmatic and controlling tendencies have intensified. The one who tarnished Seungwoon's reputation was his only daughter, Jihye, and even more so Bio, whom he considers to have no talent and only greed. Both mother and son have been stains on Seungwoon's life, and he has ignored and hated them his entire life.