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).
David-O 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
Tetsuya's song at the beginning of the story is Follow You by Bring Me The Horizon. I think it best captures his somber feelings of being directionless. He feels lost in a strange new world and he has no real goals of his own to push him forward. It also shows the romantic undertones in his relationship with Chelsea.
Chelsea's song is either Centuries by Fall Out Boy or RISE by I, Prevail. Both songs have similar themes of being ambitious and chasing after greatness, so they're kinda interchangeable. Chelsea is very goal-oriented and the entire story is basically about her chasing after her dreams.