So I am slowly re-reading a favorite of mine - Hugh Cook's Wizard War.
One thing I like about this book is the way characters are introduced. It's not a bad model for a stat-free character sketch for games, either.
Let's take one of the wizards of the book, Heenmor.
Status: Master wizard of the order of Arl. A renegade wanted dead - most definitely dead - by the Confederation of Wizards.
Description: a massive, troll-shouldered giant, twice the height of any ordinary mortal. Black eyes, blue beard and ginger hair. Robes of khaki, boots of white leather.
Career: most notable exploit was his organization of an expedition to loot an artefact of power from the Dry Pit [. . . ] His companions either died in the Dry Pit or were murder by Heenmor afterwards [. . . ]"
- Hugh Cook, Wizard War, pg. 14-15
It's a punchy description and you can get pretty far with that in knowing what he's like. Your guesses from that won't be far off, either, in the book. Or more importantly, your guess at how to run him in game. A whole plot is encapsulated there, and so is his nature, his level of power, and his unusual size and shape.
How about another bad guy? They read well.
This is a prince of particular importance:
"Name: Johan Meryl Comedo, prince of Estar.
Occupation: ruler of Estar.
Status: Class Enemy of the Common People.
Hobbies: preservation of traditional royal prerogatives by way of rape, torture, looting, arson, sundry oppressions of peasants, incarceration without charge of trial, etc., etc.
Description: not quite the man his father was."
- Ibid, p. 44
I don't know about you, but I know this guy already. I immediately have a mental image of his effete and treacherous face, his whiny edge to his voice, his reaction to just about anything someone can do in his presence, if he even bothers to notice them. And we know what's he's not living up to, and how everyone around him secretly feels about him, whether they serve loyally or not.
Having a loose structure of this sort, highlighting the really evocative and necessary details, seems like a good possible way to organize your notes about NPCs. Or present them to others - if you're writing for publication, you need to convey your impression of the NPC to someone else to run it in game. The advice ("Write done evocative stuff about NPCs") isn't new, but the presentation might be new to you.
For example, here is one I just whipped up for Father Hans, the NPC healing cleric in my game.
Name: Father Hans Goodman
Occupation: Priest of the Good God
Status: Healer of the Second Rank, currently employed by Dryst.
Description: A middle aged man with brown eyes, brown hair with a bald spot, a forked beard worthy of remark, and a club studded with silver nails.
Peculiarities: Addresses everyone as My Son, and remarkably unsympathetic to others' calamities - the Good God does work in Mysterious Ways.
That works pretty well for me.
If you get a chance, read that book, too. It's very entertaining. It seems to be part of a series, but the supposed next book in the series was a light comedy and I couldn't get through it. This one is much darker, and very, very cool.