Your main character's theme song
Also it's okay to list one or two extra songs if they're for different moods or scenes with your given characters.
A few songs for my story are:
Jericho King: Unstable by Chaotica, the song fits him pretty well given his general mental issues and how he still tries to do his best as a Mask.
Graves & Molly: Friend like me from Aladdin (1992), whenever Graves is trying to cheer Molly up, it just fits given his status as a powerful magical entity and his love of music.
Aiden Caine: Sinner by Barns Courtney, just in general for where he starts in the story and the person he's trying to be.
Song: Burning My Soul from Kamen Rider Build, because like the theme song of the character in the show, Song is a fighter that's all brawn and no brains, but somehow he always feels so reliable when he gets to work.
Shu: Revolution from Kamen Rider Ryuki, because of its tragic yet heroic vibe. He'd risk everything to fight and protect Song. The both of them share a tragic story as well which makes it even more fitting.
Aqua - Aquarius. While this song is purportedly about a drug addiction, which she doesn’t have, the lyrics fit her mixed emotions about her transformation and MC Tentachill.
Samurai Jack OST - Jack in the Rave. He’s loud, he’s obnoxious, and he uses music to control people (sort of).
Wrote: Wasn't this (theme song thing) a whole episode of Ally McBeal back in the day?
No idea, but I've seen similar things pop up where people give new theme songs to their favorite characters from a show, game, or movie, and figured it'd be a cool idea to see what other people thought of their own characters.
Especially since I usually use said songs to help my stay in character when writing, from the many perspectives in my story.
"I Cain't Say No" - R&H's Oklahoma!
audiafauna - Grow Down