When running a PBEM, or any role playing game for that matter, NPCs (non player characters) are critical story telling tools. Good use of NPCs can entertain the players, bad use of NPCs can make a group quit en masse. So the first thing you need to figure out is, what are the NPCs there to do for your game and your players?
First of all, an NPC should never overshadow the player characters, meaning you should not often throw NPCs into the game that make the characters look weak or ineffective (the exception are villains, but I will touch on that in a later post). For example, if you are playing a DC Comics game when the players are gritty, street level vigilantes, it would not be very much fun to have Batman showing up every few weeks to remind them how much more awesome Batman is than they are. Use of Batman sparingly might prove to be interesting; he may require their help, or the heroes manage to thwart a Batman villain and earn Batman's respect, or they rescue Batman. Maybe they rescue sixties Adam West Batman. The other versions? Not likely. The point is a more powerful, more skilled or otherwise more formidable NPC can put a damper on your campaign very quickly if used incorrectly.
This is my opinion and I understand if people disagree with me on this point, but I do not think PCs should play second banana to a more powerful NPC. For example, if you are playing an Indiana Jones game, who wants to play Short Round or Sallah while the GM role plays one of the greatest characters in cinema? Even after Kingdom Of The Crystal Skull? I once saw someone (repeatedly) advertise a game on the old PBEM.com web site, where he was going to run a powerful super heroine along the lines of Wonder Woman, and he wanted players to play her supporting cast. Considering the complete lack of takers I think it spoke volumes as to what players want in a game.
Second, an NPC should not make a Player Character redundant. Say you are running that super heroic game and you are playing the team brick, only the GM has introduced a hero who also has super strength. Suddenly your character seems just a little less awesome. Or necessary. If NPCs exist on a team they should fill a void, fulfill a need. For example, when I was running my first super hero game I noticed the team had no brick, so I created one. I also noted there was no doctor, so the brick was designed to be a geneticist and surgeon. When I began my second game both roles were filled by at least one other player, so I swapped out the NPC for one more useful; there were no technical specialists so the NPC introduced was handy with gadgets. And many of the team members could not travel quickly, so she had flight powers and could create force fields to carry them.
Now, you may think that a gap in the team roster might make for more interesting role playing and I will concede that is a valid point. How does a cyberpunk team accomplish the mission without a hacker? How does GI Joe take the bunker without Bazooka on the team? A skills gap amongst the PCs can be a challenge and it is up to the GM whether he wants to make the players work it out or provide them with a skills patch. But I think that the continued gap might prove to be a pain in the ass for the players if they keep going on missions/adventures that the GM insists needs personnel/skills they are lacking. The GM could run the risk of going the opposite way, running multiple adventures that make his pet character indispensable. NPCs should be useful, but not annoyingly so.
Third, NPCs should make the game run more smoothly. I am not saying they should make the game easy, only there are situations where NPC involvement could make for a more satisfying role playing experience for the players. In cyberpunk games, for example, the hacker may sometimes enter a virtual world and do battle with security systems. The cyberscape environment can be an exciting place as the hacker combats corporate hackers, killer AIs (Artificial Intelligences. I do not think my readers are stupid, I just do not like to assume everyone knows every little bit of sci-fi/fantasy jargon I do), hunter/seeker programs. It can be very cool. Problem is, what are the rest of the PCs doing while the hacker PC is having the time of his life shutting down security for the infiltration team? A fight like that could take days, if not weeks of time as player and GM exchange e-mails. Maybe having the hacker an NPC and fast forwarding through all that stuff would make things easier?
NPCs should never take up so much space that the players feel like they just belong to the GM's fan fiction message board or mailing list. GMs should avoid constantly having NPCs talk to one another without PC input. In my Vindicators game I do have instances where there are interludes, moments where the PCs are given glimpses into what NPCs are doing elsewhere. But those interludes serve to provide a set up for future encounters and are not an end in themselves.
There is also a type of NPC called the GMPC, and these are characters the GM are so fond of they are featured in the game as much as the PCs themselves and given far more depth. GMs are often frustrated gamers, and employing GMPCs is a way to scratch that player itch. GMPCs are fine, provided the earlier rules about NPCs are adhered to; GMPCs should not make PCs feel like they are the GMPC's sidekicks, nor should they make them feel useless. One exception to this rule would be Star Trek. In Star Trek games the captain is played by the GM, and quite often the captain is portrayed as being in some ways superior to the the PCs; he is captain, after all. He achieved his rank through bad-assery and derring-do (although a captain who earned his rank through sheer nepotism might be really interesting to see). This is done to maintain game stability. Sometimes players leave and the GM has to explain their absense (Or not. I have seen games where character absense is not addressed at all, as if they never existed), cover for the gap. In a Star Trek game if someone is playing the captain and they quit that means the ship gets a new captain. Suddenly. Out of the blue. So while a ship can survive constant engineering officer turnover, repeated captaincy turnover stretches the boundaries of the players' suspension of disbelief.
NPCs who appear often should be fully realized characters. Even Alfred Pennyworth has a back story, and sometimes that story gets the Caped Crusader involved in an adventure or two. NPCs that directly affect PCs should feel more like real people. Your players will appreciate if you give their loved ones or rivals a third dimension; I am not a big Harry Potter fan but when I saw The Deathly Hollows I felt Draco Malfoy suddenly became a tremendously more engaging character when he was given depth; no longer was he simply Potter's annoying school rival, but he was now an unwilling pawn compelled to do horrible things. He became a fully realized human being with motivations and fears all his own, and I thought in that instant if there was any character who could sustain his own series in the Potterverse it would be Draco. That is sharp writing, excellent direction, and damn good acting. Now imagine if your PCs had an enemy who gradually goes from being a two dimensional baddie to someone complex. Star Trek, Deep Space Nine's Gul Dukat was a fascinating character who started off as Sisko's annoying nemesis, but over time grew into a complex character...Of course, they completely screwed that up when Dukat took the train to crazy town, but at least he was still interesting.
I will address the roles of antagonists in a later post, Depths of Villainy. For now think about what I said about NPCs; they should be useful, interesting, and almost never overshadow the PCs.