Many GMs do it. Many don’t. Sometimes a group needs the rounding out or a sanctioned event isn’t considered playable without it. Sometimes you can’t find enough players but want to try to have a campaign anyway.
The Party NPC.
There are a lot of pretty great reasons to consider the in-party addition of an NPC. There are also some really terrible risks you may take in choosing to make the inclusion. When evaluating the possibility of the Party NPC keep the following in mind:
Pros and Cons of “Party NPCs”
Party Balance – Hey look, the awesome role-playing heavy group you have didn’t want to break their backstories enough to have anyone one who can notice or disarm a trap. Not always an issue but you did mention you were only going to have time to run modules this month right? The kind that assume traps and rogues? Buehler? This scenario is a classic reason for the Party NPC. It also can be a great idea for when the group needs a “fifth wheel” role (even temporarily) to handle a situation. What do you mean no one in the group has water breathing? Well the undine in the corner over there looks likely and very bored…
Story Needs – Sometimes the quest for the heir-blade of the ancient human empire Navaelus is one of the coolest game ideas you have ever come up with. And then everyone in your group walked in with demihumans and none have even a shred of human heritage. Maybe they are the last heirs protectors. And the heir is going with them to reclaim it. Maybe one of the party members has family or friends in their history and no one was willing to play it. Maybe the party needs a trojan horse in it. Maybe someone needs to die.
Info Dispenser – Nothing halts up play like a stumped party. Sometimes the Party NPC can be the pump that gets the action going again. A hint or clue or even a corrective statement can go so far to fix a derailed party and get the players back on the right course. And hey, how fun is it to have someone who can be really WRONG with the players or LIE. Oh yes. Mis-Info Dispenser is an equally valid and wonderful reason for the party NPCs.
Mechanics Previews – Someone asked you if you would allow a new third-party class in your game or a new spell or well anything. Give it to the Party NPC and see how it plays. You can run it with abandon and a single-minded auto-fire a real player would get bored to death doing. By the end of a few sessions you should get a pretty good feel for it. This is particularly great if you run multiple games where you can preview things without “spoiling” them for a player.
Spotlight – If your group is already big enough adding a force that competes for screen time can be a bad idea. Believe it or not, a great NPC might interest players more than their friends characters do. Remember this problem and make sure that if you choose to use a Party NPC they aren’t too limelight intensive.
Solo-Session – A variant of the “Spotlight” issue, this situation is one where an extended NPC/Player interaction excludes the rest of the party. I have had players get very frustrated with this and with players that seek to make it happen. Make sure that regardless of the NPCs reasons for being there, they are aware of the other party members. This doesn’t mean the Party NPC has to be a sycophant or pleaser, negative attention is attention too. Just be be careful not to split up the party with too much tension if it is mixed between negative and positive.
Superman, Gandalf, and other Saviors – Don’t do it. I know sometimes GMs don’t get to play enough. I know sometimes we want to see a new build in action. THE biggest complaint I have ever seen was the slew of “OmniNPCs” and re-skinned Mary Sues that GMs put into their own worlds and stories. Yes, NPCs should be meaningful and a Party NPC has to have enough skills to survive but nothing will make a group feel more meaningless that a Party or frequent NPC that is always trumping an/or bailing them out. Really watch this. It can be terrible.
Processor Lag – Oh look another thing to level. Another viewpoint to consider. Another “mouth to feed.” Sometimes I think this is the biggest reason why GMs DON’T use a Party NPC and it is a pretty valid one. Nothing is quite frustrating like running a large combat, with a Party NPC (or two), an a couple absent players… it can be like spinning all the handles on a foosball table, poorly. If you have a doubt as to your ability to manage and invest in the party NPC it might not be a good time to use one.
Whatever you decide, the presence of the Party NPC should always serve the characters, story and make life easier overall, even if they raise a little hell getting there…