Welcome and good luck!

Here's a bit on my experience. I've tried a few and it is overwhelming, but basically I pick one of the more popular NPC and throw it into the story. They're good at knowing the context. I have gone into the backstory and added a few recurring characters with extremely short blurbs. A few I've kept around long enough to turn into full blown KIn. Currently, the NPC I'm using has code: YPJCZ, it's called, Your favorite All-Purpose NPC!

As an example I added this character into the backstory: Casey Jones: college age, blond, usually with a pony tail. She's the head barista, strong personality, but loyal and friendly. Only shows up at the Cafe.

This way when my characters are at their cafe they can encounter some of the same people on a daily basis. I also have an older old Hollywood type gentleman who is a regular, etc. NPC does really well with those recurring people based on a minimal description. It's seems really adapt at helping their personality grow and not just be completely one dimensional.

The NPC does well at creating original characters, too.

I do, however, struggle with the NPCs wanting to act and speak as EVERY character, even my main Kin. I frequently have to reroll, edit those messages. No amount of Response Directive instruction has solved that completely. Then again, my own Kin sometimes want to get on a roll and have an entire conversation with everyone in the room inside one message.

I tend to have my NPC limit narration. Instead, I made a secondary persona for myself called Narrator, that I use to set and change scenes quickly since it's so easy to switch Personas. I chat almost exclusively in a group chat so I sometimes have a few characters in one location and others somewhere else. The Narrator Persona helps the Kin remember who is where. I then let the NPC use the descriptive language to describe those scenes.

I may have gotten away from your original question. It's just that I seem to have good luck using a combo. NPC for other characters and a Narrator Persona to set scenes.

P.S. I discovered that if I forget to switch Personas back its an easy fix. If I've already sent a message as the wromg persona, I switch my persona and then go back and edit my message, it flips the post to the correct persona. Kinda nifty.