I don’t really write fanfiction (yet) but when I’m working on characters I tend to listen to music genres I think fit them or their general vibe! I don’t have specific playlists for each fankid, so usually I’ll just pull up a general playlist of whatever gets me in the right mood.
For example, I tend to listen to a lot of modern Japanese folk music when working on the Johto or Kanto kids and I’ll listen to European folk-inspired music when working on fankids like Cyril or Florian.
One of the exceptions is Hymnia, who I do have a few dedicated songs for. Some you can find on the bottom of this page. And this is the other big one. There are some other characters who I do associate specific songs with… but with most it’s not strong enough that I always listen to that music when working with them. I just brought up Hymnia as an example since I can easily link some of her “Character Songs” lol.
If I’m not listening to music, I might put a Netflix rerun or some Youtube video on in the background as I work – but this is usually just for drawing! I find it easier to write when I don’t have to try and focus on the words I’m writing and what I’m listening to.
In short, it really depends on what fankids/characters I’m working on, what kind of plot element I’m working, and the mood I’m in. There’s no real consistent answer.