Favorite NPC

Larcun the Blue
The very first 3e D&D game I ran was at WotC, with some folks from the company. We started at low levels, and when the PCs had enough money to be interested in buying some magic items, they discovered the most famed artificer in town was Larcun the Blue. They went to his shop and discovered he wasn’t in. But his apprentices were happy to help the PCs with their needs.
The campaign went on for a bit, and moved to a new area with a new city. And there the most famous artificer was… Larcun the Blue. They went to his shop and discovered he wasn’t in. But his (different) apprentices were happy to help the PCs with their needs.
The PCs never managed to meet Larcun the Blue, but they became wildly curious about him. Did he have a franchise of magic item shops? Was he even real? Did the apprentices use his name to deflect efforts at divination on who owned the shop? The character was much, much more interesting because the players never met him.

It’s a trick I’ve used other times as well, or a variant of the trick where an npc is mentioned far more than they are on-screen. Cantor Mazeborn, a minotaur cleric, is one of my most successful recurring villains because he almost never recurred. Player would hear about him messing with allies of theirs in different places, or run into his agents and allies, or just have weirdly bad luck they attributed to him. Unlike Larcun, Cantor *did* actually show up, but only rarely in a situation where combat was a realistic option. My minimizing his actual presence, I was able to both keep him around longer (those PCs were *very* good at killing things), and make him seem more competent and mysterious.