What is the character?
An errant follows one goal, and he follows it absolutely. That alone isn't so unusual -- all characters have goals -- but the errant has honed a skillset specifically for fulfilling that goal, and there's no ambiguity about when he will be done. A scholar would be happy to continue acquiring knowledge for the rest of his life, but the errant only wants to know what's pertinent to completing his self-imposed quest, at which point he'll... well, he'll figure that out when he gets there.
If it's not obvious, the errant's quest is both finite and self-imposed, and it creates a pretty sizable problem for him once it's finished. After all, if you've spent 30 years of your life training to kill one man, by the time you finally track him down and kill him your entire set of skills is rendered completely superfluous. In a distant sense, the errant knows this, but as long as he's on the road toward completion he doesn't bother to think about it. The next step after success will get sorted out when it arrives.
What's the angle?
Technically, an errant is on an adventure by definition. But adventuring in general fits well with the archetype -- there's a lot of exotic training and knowledge that you can find in the great big world. More to the point, it's often not as simple as just walking off and finishing your quest of choice, and the errant needs to learn more before he can even start in on where he plans to go. (To use Roland Deschain as a prime example, it's not until the third book of the series that he even starts heading in the direction of the Tower -- and the fourth makes it clear he's been building up to this for a very long time.)
Whether or not an errant is necessarily happy to be sidetracked depends a lot on the individual character. He might be unconcerned with the amount of time that other matters will take up in his life, or he might get angry at what he sees as increasing delays. In all cases, though, he'll stay with others if he thinks it's going to ultimately bring him closer to his goal. Unfortunately for his companions, he may also view them as completely expendable if that would mean achieving his overall aims.
What makes it interesting?
Errants have all the surety of paladins and partisans without any moral superiority, real or implied. And freed of concerns like right or wrong, he has a sort of definite progression that is either inspiring or downright terrifying. An errant questing for revenge (as many are) is long past the point of asking whether or not his revenge is justified or even reasonable; he simply sets forth on his trajectory and follows the path he's laid. And more often than not, he's willing to sacrifice pretty much anything to reach that goal, which means that there's an extra level of conflict between him and his companions.
All of that, of course, is the lead-up to when the errant finally does complete his quest -- which opens up a whole new can of worms. From there you could have a character who grows disillusioned, isolated, or even just moves on to a new objective without pausing to think. If he's unlucky, he might realize before he's finished that his goal is either unreachable or undesirable, leaving him aimless without even having a victory to show for it.
What should I keep in mind?
The reason errants are so problematic is that you are going to have to plan out what happens when they reach their goals. That means when you create one, he needs to have a clearly defined destination, and you have to start thinking about what attaining that goal will mean. Remember what I mentioned last week about not being a storyteller? With the errant, you have a setting in which you have to give someone the semblance of an arc while not locking yourself into anything.
For this reason, in-game goals work best, unless you want the character perpetually chasing a phantom. You'll also want to think ahead about what will happen when and if that goal is attained, with the knowledge that ultimately, the errant is so focused on a single goal because that's part of who he is. There's something in him that refuses to let go, and whether that's for noble reasons or just a petty refusal to move on is up to you.
For those of you who've missed the previous columns in the series, this is the sixth archetype discussed, preceded by Soldier, Rogue, Scholar, Paladin, and Partisan columns. As always, I'm eager to hear what you think, so feel free to leave a comment in the comments field or send a message along to firstname.lastname@example.org. Next week I'm going to talk a little bit about crafting dynamic scenes and taking opportunities to roleplay in unusual circumstances.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.