Piano prodigy Kosei Arima dominated various music competitions and has become famous among child musicians, but also controversial. After the death of his abusive mother, who was also his cold hearted instructor who forced him to play the piano emotionlessly, he had a mental breakdown at the age of eleven while performing at a piano competition which resulted in him no longer being able to hear the sound of his piano.
I Hereby appoint Friend A as my accompanist!
Kosei hasn't touched the piano and views the world in monochrome, without any flair or color, Kosei ended up leaving the piano for good to live life with his good friends, Tsubaki and Watari, until, one day, a girl changes everything.
At first Kaori says that she is anaemic and just needs some routine testing, but this is revealed to be a lie when flashes of her past reveal her collapsing and bleeding many times before.
Kaori opts for a risky surgery that may result in her death if it fails, just so that she can play with him one more time. During the finals of the Eastern Japan Piano Competition, Kosei starts to play the piano, wishing that it would reach Kaori
Kaori eventually dies due to the surgery but leaves a letter to Kosei, which is given to him by her parents at her funeral. The letter reveals that she was in love with Kosei the whole time and that her supposed feelings towards Watari were fabricated in order to get closer to Kosei without emotionally injuring Tsubaki, who also has limerent feelings towards Kōsei.
Hey, you! I mean, Dear Kousei, You were just here, and now I’m writing you a letter, kinda weird, right?The first time I ever saw you....