On Writing & Roleplaying Mysterious Characters
What makes a character mysterious is simple in theory - we believe that there is something about the character we don't know about. That's it. To be mysterious, there must be something about the character that raises questions that don't have immediately obvious answers or doesn't seem like it can be easily explained away by something ordinary.
However, making a good mysterious character can be a bit harder. Some people aren't entirely clear on just how to build up mystery around a character. Sometimes they try to build the mystery up in a clumsy or ineffective way. And sometimes they get fall into a trap of thinking that mysterious characters must have certain traits. So we're going to look at various ways to effectively create and portray mysterious characters, address some of the misconceptions that people have about them, and how one can build up a sense of mystery around one's characters.
General tips for creating & portraying mysterious characters
Remember that "mysterious" does not have to mean "silent," "grumpy," or "broody." Mysterious characters can behave like this, but they can also be as talkative and friendly as anyone else. The thing is, the bulk of what they say and do simply doesn't answer the questions your audience has. (Also, keep in mind that characters who are nothing but silent grouches often end up being unlikeable, and most people don't really care that much about finding out the story behind someone they don't like.)
Remember that "mysterious" does not always have to mean "secret tragedy." People can have a lot of reasons for not wanting to talk about about something. They might feel that to do so would create awkward complications or raise questions they just don't want to answer. They might feel that talking about their pasts might get them into trouble in some way - if not with those they're with, then with others. They might feel embarrassed or ashamed. They might even just feel like they have more important things to worry about. This isn't to say that your mysterious characters shouldn't ever have tragic backstories, but rather that you don't want to limit yourself to just one type of backstory for your mysterious characters, lest they start feeling like cookie cutter characters.
Be careful that you don't try too hard. There are a few common ways people end up trying too hard to build up an aura of mystery. One way is by trying to force people's attentions toward something "mysterious" by describing it over and over, or by having characters spend an inordinate amount of time talking about how mysterious it is (especially when it's not actually that much more mysterious than anything else they've encountered!), or by having characters talking and thinking about it when they really should have other things on their minds. Another way is to include a cover-up that makes no sense. Another one to watch out for is making a big deal out of how "nobody knows the truth!" or how "nobody found the truth and lived to tell the tale!" - especially if the reason for it turns out to be a bit underwhelming - EG, "because they hadn't invented science yet" or "because they tripped and fell off a cliff." And lastly, making a big deal out of a mystery the audience can probably figure out (EG, the baby left at the doorstep of a famous superhero's doorstep being the superhero's biological child) is something to avoid, too.
Characters who deliberately withhold or obfuscate information for no other reason beyond looking mysterious run the risk of looking pretentious and snobby. Stop and ask yourself: what would your "mysterious" characters actually stand to lose (or think they stand to lose) by just telling everyone the truth or speaking in a way everyone can clearly understand? If there's no reason beyond "they wouldn't look as mysterious," you might have a problem unless you're deliberately trying to make your characters look at least a bit pretentious or snobby.
Never try to make your characters "mysterious" to cover for poorly-developed backstories. You should already have a general idea of what it is that your characters aren't sharing with others. You need not have each and every intimate detail figured out at first go, but you should at least have worked out basic the framework when you begin your story or game.
Give out bits and pieces of the truth over time. If you just toss out hints that a character character has an interesting origin or past but never actually reveal anything, people are eventually going to lose interest. On the other hand, a character whom people learn everything about is no longer a mysterious character. What you can do to avoid either problem is to give out bits and pieces over time, and to hold off on giving out everything until you're ready for your character to no longer be considered mysterious.
If you're doing it right, you should never have to say that your character did or said something "mysteriously." If what your character said or did is really and truly mysterious, then it should be evident from the context (EG, other characters are confused or intrigued, or what happened is obviously unusual to the audience even without seeing how others react) and should not need the word "mysteriously" tacked on after it. (If you're looking to convey a tone of voice, something like "said melodramatically," "whispered," or "said in a hushed voice" can work.)
Ways you can build mystery around your characters
Have your characters refer or allude to past events. For example, a character might mention "the pancake incident" without going into full detail - especially in a tone that suggests something terrible or shameful happened. Or a character might observe that a particularly strange situation is reminiscent of "this one night back when I was working near Bend..."
Have your characters own, do, or say things that imply some sort of unusual experience, knowledge, or origin. For example, a well-worn book on the shelf titled How To Summon The Spirits Of The Dinosaurs might raise a few questions. So might a character being found soldering together some sort of gizmo unlike anything anyone's ever seen. And so might a character off-handedly mentioning how back home, nobody went outside at night without being armed for fear of being carried off and eaten by the birds.
Use their appearances, if applicable. For example, in a town where everyone wears red, someone milling around wearing blue might make people ask questions - why is this person in blue? Are we looking at someone from another place? Or does this person just want to try something different? Likewise, a figure dressed in a cheap Halloween costume at a fancy masquerade ball is going to raise some questions - just what is this person doing here, and why such strange attire? Of course, your characters' appearances should ultimately say something important about who they are, too - they shouldn't look unusual merely for the sake of drawing attention!
Have your characters do things that seem strange given what we know about them, or just seem strange given the situation. A character shown to be brave (even fearless!) in the face of mortal peril nervously insisting that the party take any road but the west forest road can make people wonder just what that character encountered there. And a character grinning gleefully in the midst of what seems to be a horrifying situation can make people wonder just what's going on with that character that such a reaction might happen.
Use other characters' reactions. You could have your characters recognized by figures whose position and statuses alone imply that something of interest went down (why does that shady government agent seem so familiar with, even friendly with Grandma?), or have people react to a character in an unusual way (why does the diner owner look so nervous every time Grandma reaches into her pocket, and why do the Hendersons, who are usually so talkative, start talking in hushed whispers whenever she's around?).
Come up with some rumors. Rumors surrounding a character can leave people wondering which - if any! - of them are true. And a "mysterious" character who is comfortable lying can even pitch in, even telling all manner of contradictory stories that will leave people guessing.
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