What's a part of worldbuilding you either created or read in a fic that became a permanent part of how you envision the world or character?
For characters, in one of my fics I made Giovanni the uncle of the Alola protagonist (I had the idea of making him the elder brother of the protagonists' mom because they're both affiliated with the Meowth line), and they've just become inseparable in my mind. How relevant their familial connection would be depends on the story, in some it's a defining part of the protagonist's background and characterization, and in others they've never met, but the link is always there in my mind.