Let me introduce you to Dale. Dale’s a new Dungeon Master, six sessions into running his first Dungeons & Dragons campaign. He’s a nice guy, who wants his players to have fun.
Dale has a problem. He needs to introduce an non-player character (NPC) who will stick with the party for a while. However, he’s heard horror stories about “DM PCs” that take over the game.
Let’s start with a quick definition. Every tabletop RPG includes NPCs. a DM PC is an NPC that stays with the adventuring party.
In my opinion, there are two times to use a DM PC:
1) The PCs need a guide. They may be entering an completely new environment, and they need a local who can identify dangers. Or, they may need a friendly introduction to politically powerful circles.
2) The PCs need to protect someone. Perhaps they’re escorting a merchant’s daughter to her wedding, or escorting a politician as he or she travels to his new appointment in a distant land, or guarding a noble from assassination.
In both cases, there are two qualities that will keep the DM PC from sucking the fun out of the game:
Keep the character in question within his or her rigidly defined role. The DM PC should be much less effective in combat than the heroes, though some contribution is fine. Also, the DM PC should not be important the plot outside of the temporary help needed to get the PCs to the next stage of the story. In other words, the DM PC should be as important to the story as the abandoned temple that the PCs explore or the boss of the local band of goblins: a vivid character, perhaps, but only relevant within one brief segment of the story, and in specific ways.
Which leads to the second point: keep the DM PC temporary. This character should not stay around for more than a few sessions. Combine this with the character’s limited utility, and he or she won’t take over the party.