I've been at this, as I've mentioned, for a long time. I can't readily count how many significant NPCs I've created: hundreds, I expect. I'm minded to present a new one from time to time. They'll be posted under the GURPS system, but the numbers ought not be too tough to parse out for those of you unfamiliar with the system.
Something in which I believe is what I call the "viewpoint NPC." Being a player, IMHO, isn't always easy. You're not really standing in Swordpoint Ravine, looking up at the ruined tower occupied by orcs, ankle deep in the late spring mountain snow. You're in my living room, balancing dice and your laptop with a plate of pizza and a can of DC. You're not going to recall, instinctively, that you've got a loop of rope over your shoulder and you're carrying a heavy packload. You're not going to perceive, instinctively, that it's getting pretty damn cold, you're above treeline and you've only got two hours of light left. You may have forgotten that your pal over there, in the fall that happened an hour ago game time -- but, in real life, happened at the last gaming session two weeks ago -- has a wrenched shoulder.
I like, therefore, to have a viewpoint NPC. He or she's almost always a grunt fighter, without unusual or arcane skills, and has a background consonant with the party's theme. The VNPC's generally self-effacing, and doesn't take a lead role in much; they're not brains or problem solvers, and don't aspire to be. I'm pleased if the VNPC is a bodyguard or sidekick to one of the PCs. What the VNPC is for, more than anything else, is to contribute editorial comment of things that I believe would be painfully obvious to adventurers on the ground, and less so to gamers lounging on my living room couch ... and so I don't need to go into third-person omniscient, which I prefer to avoid. "Sorry, boss, but I don't like it. Arkis can't put his full weight on his shoulder, it'll be full dark before we'll be more than halfway up that wall, and the wind's picking up something fierce. Want me to start pitching camp?"
Kardo -- the disreputable fellow lounging above -- is the longest-running VNPC I've ever had: he's been around a full decade now, the Ally of my original Quincy group's wizard, and following her around from just-out-of-the-academy to being the greatest PC mage of my campaign's history. He's undergone a lot of changes over the years, but this is more or less what he looked like in 2003:
ST: 12 DX: 13 IQ: 11 HT: 12 Per: 11 Speed: 6.25 Move: 6
Advantages: Combat Reflexes, High Pain Threshold, Night Vision+4, Rapid Healing+1, Reputation (+2, as badass pirate, among other pirates, wharf rats and lowlifes)
Perks: Improvised Weapons, Naval Training, Weapon Bond / favorite cutlass
Disadvantages: Code of Honor (Pirate); MMA/Rheumatism; One Hand; Sense of Duty: the "crew"; Social Stigma: Second-class citizen; Struggling; Stubbornness, Trademark
Languages: Avanari (is illiterate)
Skills: Area Knowledge (Warwik City)-12; Area Knowledge (Eastern Avanari coast)-12; Artist (scrimshaw)-10; Boating-13; Brawling-14; Cooper-10; First Aid-11; Gambling-10; Thrown Weapon: Knife-14; Leadership-11; Scrounging-12; Seamanship-14; Shield-14; Shortsword-15; Smuggling-12; Streetwise-12; Weather Sense-12; Singing-12
Quirks: "I am a tattoo artist," "Old Salt," Scrimshaw connoisseur
Raised a cooper's son in the stereotypical Small Outlying Village On The Coast, he ran off to sea at an early age. Many years of vicissitudes aren't pertinent, but he eventually became a pirate, and a successful one. Unfortunately, he lost his (off-)hand in a foray, and feeling he couldn't keep up any more, retired to the waterfront of the capital city, where he befriended the family of an inn catering to pirates and rogues. Their elder daughter became a wizard, and he promised her (now-deceased) parents he'd look after her. He's done that ever since.
Kardo's a stereotypical, sardonic Old Salt Pirate, and social graces aren't his mug of grog. (He prefers tea to grog, as it happens.) Even deep into middle age, he's still respected by other pirates and those who know who he is, and he can still bring it in a fight. He's a sword-and-board fighter, preferring his favorite cutlass above all. (It does take him a long time to get his shield settled and on, at least a couple minutes unassisted; any sudden fight, and he won't bother.) If you're in his "crew," so to speak, he's very loyal to you. If you're not, well ... He's not well off, and doesn't own more than he can carry. He has some underworld contacts, which he uses when the wizard reminds him of them.
About his oddest quirk is that he fancies himself a tattoo artist, and carries a comprehensive tattoo kit, complete with several colors of inks. (His tattoo skill, in GURPS terms, is 7 by default, which is quite poor.) If he has the time or opportunity, he'll tattoo prisoners with insults taken from prepared stencils, which are about as legible as you'd expect.
There you have it.
For those of you unfamiliar with GURPS, a few explanations: that Night Vision level is "pretty decent night vision," as opposed to "sees like a cat." High Pain Threshold means that he doesn't suffer particularly from shock, and wounds don't slow him down unless he's unconscious. Rapid Healing gives him big bonuses to health for day-to-day natural recovery. Weapon Bond gives him +1 for *that* particular cutlass, and no others. Naval Training gives him surefootedness on a swaying, bloody deck where others would take heavy DX penalties. Struggling means he's a bit on the impoverished side; in Kardo's particular case, at this stage in his career, he doesn't own more than he can put in his sea bag. The rheumatism? Give him a HT daily, bonuses on hot dry days, penalties on cold wet ones. If he blows it, he's -2 to everything. Ow.
For further explanation of system stats, check out this link.