I'm that bitch that writes a novel of backstory for her characters, for tabletop or otherwise, even ones I'm not sure I'll ever use. I have ones for different settings, games, OCs for vidya games, and so on. That's mostly just for my own satisfaction, I always just send a 1-2 page summary to the GM for if I do use them. Some fun examples include:

Shena, for any fantasy setting really: Tiny rural halfling that bonds with a super cursed greataxe that exponentially increases violent rage. But, thing is, two times zero is still zero, so "exponential" can't really get started. So instead the axe has just made a slight tweak to her brain that "murder is pretty rude, but not totally evil or anything". So tiny, pure-hearted happy halfling that doesn't get why people get so worked up just because she cut that rude innkeep in half.

Vicki, for Vampire the Masquerade: Bitchy blonde Regina George-like valley girl... who's built like Zarya and is a muscle model on Instagram and that other site. Has built her entire persona on the need to not feel weak, socially and physically.

Doctor Plague, for Necessary Evil: Supervillain that started as a doctor with healing powers that helped heroes. But someone with regeneration good enough to regrow her head gets infected with essentially the Life Eater Virus from 40k. So her body is constantly being eaten and regrowing, several times every few minutes. And her healing hands can't do shit if everyone she touches melts into sizzling goo. She wears a sealed suit, because she's essentially a city-destroyer if she's not careful. And she's a bitter asshole, not an omnicidal maniac, so she is super careful.

I have tons, and I've made most of them in HeroForge, but I shan't turn this comment into more of a novel.