I've actually sort of thought about this before. I've thought about themes for all of my characters, actually, because I've always kinda wanted to compose my own music - especially for comics.
(Can't go too into spoilers, bc I'm talking about characters for a comic I've yet to release)
I feel like the theme for my comic's antagonist, Varan, would be written in such a way that it can be viewed through his perspective. Like, he's that type of villain that does the type of things he does because it's what he believes is right, but in the process, ends up losing everything he's ever cared about. I'm thinking the beginning would sound sort of like a hero's theme, and as the song progresses it starts getting more and more intense, chaotic, and evil-sounding as my main cast keeps foiling his plans and he starts getting desperate for the results he wants, until the end, where it slows down to a degree to represent that he's lost everything he cared about, but especially the respect he once had from his daughter (the protagonist).
If I do end up making my own music, I'd imagine all of the major characters would have a leitmotif of sorts, like an instrument that plays through their themes and represents them. I feel like Varan's would be some sort of percussion instrument, while at the end it would be a violin (since that's his daughter's leitmotif as well, haha) or a music box for... reasons I can't say due to spoilers, haha. The song itself would honestly probably be the longest in the OST, I'd imagine it span maybe 7-8 minutes.
Anyway, I really have to pick up music production sometime because I can write a whole novel for what soundtracks I could imagine making for my characters, jesus christ.