Quirky non-player characters are the bread and butter of any good game, and if they are properly employed throughout a story they can make even a mediocre campaign great. They have all of the elements of a running gag, inside joke, and can serve as either an ally or a foil (in some circumstances both at the same time). Not many things can make a GM happier than to begin to describe a familiar person and have the players exclaim “AH, CRAP! THIS GUY AGAIN?”
A related kind of recognition is finding an unlikely person far from home. For people that have played in my Slothjemian setting they know all too well that you can find another Slothjemian in the most unlikely places. Errant knights, both male and female, have a tendency to show up in a story with little or no fanfare. They are almost always equally surprised to find the players there, too. Sometimes they team up to accomplish a common goal, but more likely they exchange information and then go their separate ways. Frequently the paths of the NPC and the players become entwined such that they will keep bumping into each other. This is how it is with adventurers. A good caper will draw attention from multiple individuals and an NPC worth their salt will undoubtedly be tempted to seek out the same fame and fortune that the players are after.
Slothjemia is the extreme example of this in the gaming world I use. The folks that live in Slothjemia tend to be more restless and thrill-seeking than other people. This is especially true of the large and ever expanding royal family. A lot of the royals don’t have official jobs or duties and are thus free to do whatever the heck they want. The further removed from the actual line of succession they are the more likely that they will be found adventuring, doing business, exploring, or just hanging out in far flung corners of the continent, planet, and even beyond. This has often served as a way to give the players a break from an alien environment feeling too overwhelming as well as to provide a built-in ally or at the least a sympathetic persona the players can interact with. This is the kind of person that Shr Rackerby Helfenstein von Slothjem happens to be in the story “Fight, Swear, Loathe.” The main characters are doing their thing and find themselves in unfamiliar, potentially hostile surroundings and then just like that there is somebody who they can relate to if only a little.
This sort of thing isn’t a constant. It is too much to expect that all of the characters in the game are connected like this. That would be asinine. Like finding out that Darth Vader built C-3P0 as a child. That is just lazy writing. Don’t be that guy. Don’t be a George Lucas. The guy you want to be is Irvin Kershner. People don’t want Ewoks, they want a successive line of admirals being force-choked in what is handily the greatest running gag in any movie ever. As a GM you are entrusted with not overplaying a great gag. Use it wisely.