This movie is adapted and based from a real life story, and describe the life of a real person, teenager named Asano Taigi. Based on what is described the movie, he is seen to be a kind and cheerful person.
He created the fight song for his Alma mater, Ichifuna Soul (which is still used until today as a cheering song) and some other pieces.
Unfortunately, this gifted person had to face cancer in real life and despite setbacks, he managed to write a few pieces of music with his gradually weaken body when he is battling with his cancer in real life. He lost his battle on 12 Jan 2017 and his friends who are attending his funeral are playing the music he composed there as a way to remember his 20 years of life.
If he still alives today, he would be an teacher on a Japanese school teaching music or even composing additional pieces of music. Despite his short life, this movie is dedicated to him to record his brilliant life, so that he can be remembered by the world.
You can find a lot of articles that are dedicated to him on Japanese internet, which you are welcome to read. Also do check up the subber Edward Wong which subbed this movie for your enjoyment.
May his soul rest in peace.