If my NPCs hang out with my players for more than a session I roll up their stats and throw them on a notecard. Sometimes a player ends up controlling an NPC for whatever reason (charmed or their PC is unconscious - or dead). They tend to fill out the character, draw character portraits and embellish their personality which I try to use in later sessions (if the NPC survives!). It's great letting the players flesh things out for you!
Here's two shining examples of what the players have helped create in the campaign.
Unfortunately, both Tero and Mor fell exploring the Tomb of the Iron God!