There's a lot of PC Characters within the world, and there's a lot of monsters that can clash on even footing with PC characters, but what there's a lot more of than either of those groups is low level NPCs filling in all the background roles that keep the world running.
Characters with NPC class levels have a few common rules:
- No Skill Ranks: Characters with NPC classes don't track individual skill ranks. They just get a number of skills to have that always count as being at max ranks.
- No Feats: Characters with NPC classes don't get feats. Instead, for each feat they would normally get the character gets some sort of fixed bonus (depending on their class).
- Simple Stats: NPC classed characters have a common stat array they use, modified by racial ability adjustments as normal.
- Non-Commoner Array: 13, 12, 11, 10, 9, 8
- Commoner Array: 11, 11, 11, 10, 10, 10
- Max Level: NPC classed individuals never go above level 5. If they should have cause to be more than 5th level, give them levels in a real class.
- No Multiclassing: NPC classes shouldn't multiclass at all. If someone is important enough to make them some sort of muliclass setup then give them levels in a real class.
- Low CR: NPC classed individuals have a CR equal to half of their level. In order to keep this from being potentially unbalanced, we strongly suggest that they show up in groups. So while a 4th level Warrior is nominally a CR 2 opponent, he's not really a good challenge for a 2nd level party. But 2 of them are an appropriate challenge for a 4th level party. And that's really what they're for: showing up in groups and getting smacked around anyway.