I've had a recent situation that fits here pretty well.

I plan out my games long in advance. Like, I typically have the overall story fully fleshed out before I even start Session 0. I'm talking villains, major story arcs, recent galactic history and politics, ect.

For my current High Republic campaign I've been running since November, I designed a Sith named Darth Ourne, Lord of Suffering. He is a Mikkian, who had been badly injured during a battle between a Jedi task force and a group of pirates known as the Perlemian Toll, twenty years before the start of the campaign. His wife and daughter were assumed dead during the battle. His ruined body was found by a Sith Lord in the wreckage, who helped him channel his hatred and agony in order to sustain his life.

His resentment of the Republic, over the loss of his family and his ruined state, lead him to take on a political position as a Senator, and try to topple the Republic from the inside (as part of the Sith Grand Plan).

Long after I'd designed this character, I ran Session 0. One of my players designed a 24 year old female Mikkian Jesi, who had an obsession with pirates. She had been found by a Jedi after a pirate raid, one of the few survivors. With her identify lost in the ship's wreckage, and her family presumed dead, she was welcomes into the Jedi Order as a young girl.

By total coincidence, my player established a Luke Skywalker-esque relationship with my Vader-like Sith Lord.