Good morning! First, if you haven't voted in the poll to the right of the page there, please take a minute to do so. As I said, think of it as helping me either win or lose a drinking bet.
Today I want to discuss a bit about limiting campaigns. One of the topics that came up in our GM's Jam this year at Gen Con was a group playing in Dark Sun had a character who wanted to play a character who was all Lawful Good giggles and sunshine. If you know anything about Dark Sun--well, let's put it in pictorial terms for those unfamiliar with the setting--
Do you see why that might not work? And I think we've all known that person who wants to play a ninja every time, be the setting Midnight, Greyhawk, or Robotech.
In any case, if a player is wanting to play something that flat-out doesn't exist in a campaign world, it can cause a number of issues, including throwing off the feel the GM is trying to set for the campaign, making the player in question feel useless or undervalued, and potentially be a distraction from the main offerings a specific campaign setting offers.
When you see this pop up, there are a few potential reasons it's likely happened:
1) The Game Master did not adequately express the boundaries of character creation in relation to the features and limitations of the world,
2) The player in question was given the expected limitations, but disregarded or did not understand them,
3) The Game Master allowed an exception for a character concept that turned out to be problematic.
I myself have been guilty of #3 in the past, as one of my most shamefaced moment as a Game Master came when I absentmindedly assented to a player's request to be a half-elf in a world that quite firmly had no half-elves. Add in the general distrust and revulsion elves were privy to from humans in that world, and you have an incident I am still teased about to this day.
If your player insists on wanting to play a pixie fairy in Dark Sun, or a satanic biker in the Forgotten Realms (actually, that might work), it's probably a given they either don't know much about the setting or expect the setting to bend to accommodate said characters. If you aren't dearly attached to the setting at hand, you might consider choosing another setting that will let them play something they want and have it fit a bit better. Or, you might just tell them it won't work, and they need to come up with something more in line with whatever it is you're running. Most issues like this can be resolved during a group character generation, when peer review and brainstorming can help decide and define the tone for the campaign to come.
Really, as a Game Master, here's what you need to do in this situation:
1) Look to yourself first; make sure you explained what the setting's all about before character generation. If you didn't, do so next time, and before you go any further, explain it to the group now. If you allowed the problematic exception, you need to own up to it, before anything.
2) Examine your campaign setting. Are you stuck on Greyhawk, or were you just running it because it was handy? Is there really no room for this character in the setting? Would it ruin what you and the other players are trying to do, be it dark fantasy, sword-and-sorcery, or light pulp? Is there room to tweak things? If not, you need to go to the individual in question and help sort it out.
3) Take positive control. Don't waste their time having them roll and run a character that won't fit the game. I think we all know it'll be a bad end sooner or later, in that case. Explain why the character idea with the setting won't work, and suggest alternatives.
4) Be The GM: If they still aren't willing to work with you and the campaign, then you need to be the Game Master you signed on to be. Talk it over with the group, if that's your style, but handle the situation. Maybe they need to sit this one out, or maybe the group will find something else. One thing's for sure: ignoring it will just make the gaming unsatisfying all around in the long run.
Like so many things, this issue can be resolved 9 times out of 10 with proper communication before the game, but whether it pops up then or later, it's up to you as the Game Master to see things put right.