Nagisa’s backstory is more or less the same as canon, except his father figure was interested in all types of musicians instead of specifically idols.
When he was younger, his father figure would show him all kinds of footage of musicians, including Star’s Nirvana, which prompted him to be interested in the competition. He wants to win, to feel and understand the love his father figure gave him.
Nagisa received vocal training and lessons in almost all types of instruments. Due to his natural talent, he is able to play almost anything to a good extent. He was also introduced to Subaru’s father when he was younger, and was given a vocal lesson by him. He admires Subaru’s father deeply, as there was some type of “shine” that was present when he sang, and he hopes he can perform the same way one day.
He was often asked to perform for his father figure and those moments were when he would be praised the most. As performing is all he’s ever known and been acknowledged for, he believes that he was born to be a musician. Despite this, he doesn’t know yet what it means to be a musician, and is continuing to perform so he can maybe one day understand the deep love his father figure held for them, and reach it himself.
After his father figure passed away, he lived with Hiyori for a while. They would sometimes sing/play instruments at parties together, much to the audience’s glee. He remained close with Hiyori as they went to Yumenosaki together, and they decided to form a band to keep in touch as they went to their separate schools.
He’s often travelled overseas, and knows a smattering of all different kinds of languages (at the expense of losing some of his comfort in Japanese, due to being away from the country for so long). He enjoys listening to a variety of music from all parts of the world.
Nagisa can often be found humming absentmindedly as he wanders around, or polishes his rock collections.
Because he was isolated from the world and exposed only to music, he sometimes finds it easier to express things through songs, and will often think, “I feel like… xyz piece/concerto/song”.
Nagisa has a special microphone, which his father figure gave to him, that he cherishes and tries to use in every performance. It’s beset with a single, large diamond.