As a writer, player, and game master one of the hardest things I've often found to handle is an extreme personality in a character. It isn't that I doubt these kind of people exist, just that they don't exactly gel with my way of thinking or seeing the world. I see the weaknesses and foibles; the places where they would falter and - in the case of most fictional universes - be destroyed. That doesn't mean that they're not fun, or that they can't be used though. In many cases, they can give the kind of shake up that a game just might need.
In the most recent session of my bi-monthly L5R game I was down a player. The player I was down is the person who often brings the game into the upper echelons of Rokugani society. His character is a noble, and so that world just gravitates with him. Down the noble, and with 3/4 of my remaining players having a stake in the underworld in some way, I decided to try for a more 'classic' session and bring everyone together. The answer, an NPC with an extreme personality and the power to handle it.
Now, the NPC wants a meeting with one of the PCs. Only, she doesn't think the PC would respond to a normal invitation in a timely fashion. So, what does she do? Well, she sends a bunch of goons to go and abduct the PC. Then, while looking for information on another matter, she goes for the Two-Fer and has her thugs abduct another PC in the area, because, well, why not? The two abductees (mostly the first) are noticed by their relevant organizations, which gets the other two PCs involved. At this point my session has two characters who have been abducted trying to figure out what is going on, why they're there, and how bad things are going to be. The other two PCs are preparing to launch a Metal Gear Solid: Rokugan mission to sneak in, kill everyone, and break the other PCs free. Just as things start to launch though, the true purposes for the abduction become clear, and everything goes lopsided.
One of the MGS PCs overhears that the ringleader just wants to talk, de-stealths, and has a polite conversation with the woman in question. The abductee PC shows up, and they all chat quite civilly with a few hidden threats thrown to each other along the lines of "do this again and we kill you" countered by "you'll try, and fail" from the Imperial. The other MGS PC ends up having tea with the captain of the guard he was about to kill. The final abductee PC ends up getting a free cat out of the encounter, and everyone gets a ride back home to enjoy the rest of their winter. Everything worked out, but definitely weird.
A Tool For Action, Entertainment, and Obstacles
I like how the session played out because it showed how an extreme personality can shake things up in a variety of ways and cause a bunch of interesting problems. The ringleader (just going to call her Otomo from now on) wanted a meeting and immediately defaulted to extreme measures to have it brought about. The PCs, as PCs, had every right to just go barging in and kill everyone if they'd wanted to. It would've made the session play out differently, and have different consequences come out of it, but it could have happened. They were cautious though because the situation wasn't fluid. It was weird and something was definitely strange with the entire setup. In a sense, they were all presented with challenges that didn't meet the normal expectation for what they'd set out to do. I mean, how many expect "tea and chat with an Otomo" to be one of the bullet points on a "I'ma sneak in, kill everyone, and save my friend" mission?
These kind of personalities can do this in a lot of other ways to. Think of the issues that could come from a character who simply defaulted to killing anyone in their way. You can do this in other ways too, just by picking a particular solution - or type of solution - and running with it to an extreme. Alternatively, over-exaggerate some other aspect of the character and let it flail about and smack someone in the face. Extreme personalities, and characters with them, tend to instigate action, and that can be just what a game that may otherwise be growing dormant needs to jolt it back into place - or to jolt 4 PCs who otherwise may not work together into cooperative action.
Everything In Moderation
The trick you will need to learn with this, especially as a GM, is to keep it in moderation. Don't run roughshod over your game with these characters. Don't let a PC with one run roughshod over the other PCs. Keep it in check, let it loose on occasion, and then put it back in the box. In a lot of ways you can use it like the Hulk from Marvel Comics. Unleash it on the big problems and it can be an amazing solution. Unleash it when it isn't needed, and it can cause a huge problem. Both can be fun to work through in their own way, but you want to know which one you are walking into as a GM before you set it loose. If only so you can steer the players into its path the right way.