The most memorable NPC we ever met was Freddy. Victorian London, Call of Cthulhu. Freddy was a member of the Drones Club (From the Bertie Wooster novels), a foppish efete lay-about who was entirely supported by his mother and so had nothing better to do than gad about town, eating and drinking his way through the cream of British society. He came from well-bred stock and so his manner and demeanor was accepted everywhere; he was the 'exception that proves the rule', as it were.
We'd encounter him pretty much anywhere in England and once or twice on the Continent (the campaign spanned about five years of real time in total). He was always a hoot, with his antics and extreme behavior. He helped us a few times, and we helped him out of some social predicaments as well; he was a club member and a fairly good friend despite his outrageously efete mannerisms.
Around about the third year we knew Freddy, we started to notice stuff. Hmm. Freddy was effectively invisible. He was a very well known public figure, but so well known for being a totally useless clod that nobody paid him the least bit of attention. He could hang out anywhere and no-one would think anything but 'Oh, that's just Freddy'. And he was a social lion. He knew everybody that was anybody, whether they would receive him through the front door or not.
We 'encountered' him in the middle of a mission around the end of the campaign, as things were ramping up to a final confrontation between us and the avatar of Nyarlathotep who was going to replace Victoria as queen and Rule The World, ushering in the final darkness. Freddy's voice drops about two octaves and he opens his sock-filled valise to pull out two huge pistols. He was, and aways had been, an operative of MI-6, who had a fair amount of knowledge about the Mythos and who was assigned (among other things) to monitor us and make sure we had no intention of ignoring the Official Secrets Act papers we signed some time ago. It was so stunning and at the same time so totally natural that it took us a couple weeks to stop going 'gaahhh' at the whole thing.