Ahh... NPCs...

We like having them in RPs. While the Antagonist type HeySeuss described is a tried-and-true formula, we personally prefer others. Mostly because our RPs don't have a single group of PCs vs. a GM-controlled enemy. We very much prefer to set two (or more) factions of PCs up against each other. In our experience, that reduces the plot workload for the GM(s) a little, since the players make conflicts for each other.

Thus, when we make NPCs, we distinguish them in certain ways. Some NPCs are nameless grunts, some are mere names with a short concept attached, and some have a true character sheet. The former tend to be free-for-all to use, whereas the latter tend to be harder for others to use. The way we distinguish the latter from a full PC, is through the length/complexity of the CS. A PC tends to have far more in the CS than an NPC.

Sometimes we'll make an NPC, only to later convert him or her to a full PC if (s)he grows sufficiently. How that growth is determined on a case-by-case basis. No single factor defines it perfectly.

We also tend to have our NPCs be a bit more mortal than PCs. We don't mind killing any character if it'll help the story, but killing an NPC should always be easier than killing a PC. We've several times made the mistake of having too many NPCs that we control in a single RP. Its that experience that taught us the necessity of discarding some after use and to limit creation. NPC-hood is a decent way to test if a character concept is viable before one puts PC-CS effort into it.