Kosei Arima is a pianist who has won many awards and recognition due to his precision, timing, and ability to switch between tempos easily, earning him the moniker "human metronome". However, he came so far to his position because he was meticulously taught by his terminally ill mother, Saki, who was abusive and demanding. When, as a seven-year-old, she slapped him after a victory that she deemed unwarranted, he declared his refusal to play the piano again and wished that she died. Indeed, Saki died the following day; this caused Kosei to experience a mental breakdown, making him unable to hear sounds from the piano as well as seeing everything as Color blindness. Kosei subsequently quit the venue.