I haven't run many games and of those most were disjointed episodic adventures. My NPCs are generally ephemeral and disposable. Not that they get sent into the grinder often like fodder, but they don't stick around. The focus is on the PCs and the parties I've run tend to travel a lot. We're into the Final Fantasy "Overworld" travel concept.
About the only ones that end up making more than a cursory impression are DMPCs (oh, "dirty word", you say). Sometimes I feel they're necessary.
In one PBeM game the players were mercenaries in a generic fantasy world but using inspiration from the Vikings Campaign Sourcebook, part of the 2E era Historical References line. I was trying revive a dwindling group with some players I knew from other games, but the originals ended up dropping out and I decided to go ahead with the new recruits. The game was 3E and I had plenty of resources to use for monsters and encounters, but at only two players. I felt the group a bit light, especially in the divine magic area.
In stepped Lothar, an elderly cleric, former campaigner, lean old wolf of a man with scraggly beard and thoroughly grayed hair. What was so special about Lothar? Nothing really. I purposely kept his stats average then applied the age modifiers. I didn't want him to be a combat machine so the party, consisting of a barbarian and a skaldic bard, could take the front lines. He stayed back to heal and boost the player characters and whacked any stray enemies with his club trying to sneak past to flank the party. Even then he mostly held them at bay with his shield until the bard and barbarian could bring arms to bear against their foes. After all the first rule of D&D is to always protect the cleric.
Old Lothar would be winded from battle anyway and couldn't last long facing any opponent meant for the two leads (anything other than a random henchman to pad the opposition with a more imposing number). The party started to interact with Lothar more, seeing him as one of their own rather than just the "healbot" role he was solely designed for. I'm not a great GM, but Lothar was one of the rare moments of running a game where a player complimented me on the character.
Maybe he stuck out in their minds as different from other NPCs or pseudo-party members who were as young and capable as they were. Maybe it was seeing him kiss his wife farewell before departing with the party on "one last mission" to march towards the present doom facing the lands. Maybe it was his expressed worry about his sons serving the jarl, besieged by the very forces they now faced. It could have been his war stories of visiting the players they were traveling in, acting as an impromptu guide. Or it could be seeing an old man struggle with the limitations of his body in a time of conflict where physical might was the currency of the day.
Something clicked about Old Lothar, something more than the sum of the traits I spun into him. That something allowed me to channel Lothar easily as if the Old Wolf actually lived. For the brief time the game lasted, it didn't matter to the characters than one of their party was an NPC or an old man useful for nothing more than providing a second pool of hit points. They were three heroes out to save the kingdom.