Almost every Science Fiction or Fantasy story has a duel of some type in it. Whether it be the two swordsman in the ruins of an old church, gun slingers at dawn, or space samurai fighting in the throne room of a battle station, the concept of the duel is something that we, as a society, seem to love. And why not? Duels are generally very high emotion, high drama, and fun to watch to boot. Is it any surprise that many players want to be duelists when it comes to their game of choice, or at least be duel capable?
The Common Traits
Interestingly enough, there is very little difference in the common traits of duelists regardless of culture. The Samurai duelists from anime, manga, and movies are very similar in key ways to the wild west quick draw artist and the noble with his rapier. In all cases, the character tends to specialize in speed, finesse, and accuracy over brute power and tenacity. A good duelist is almost always the guy with fast hands and good control over his entire body. There is no wasted motion on power swings, or wasted energy in trying to catch the person fleeing from them. Everything is smooth, quick, and precise.
Aside from this physical trait, duelists are also almost always quick of mind and strong of will. They have a confidence, often a quiet confidence, about them that carries them through life. They don't back down from opposition, and are sure of their ability to take down whatever lays before them. This confidence and speed of mind is needed for the fights they are regularly getting into.
With their life focused around the duel, is it any surprise that a duelists weapon is almost as important as the character itself. Generally, the weapon reflects its owner: fast, accurate, and elegant. You don't see too many duelists running around with claymores on their back, and you don't see too many gunslingers using shotguns and not pistols. The smaller weapon: the katana, the rapier, the pistol, serves as a reflection of the owner and helps convey that same message. It isn't size or brawn that matters for these people, it is the skill with which they wield the weapon, the accuracy with which they can employ it. Deadliness is a combination of those two traits, not something as inelegant as brute force.
As a tip, if you are playing a duelist take some time to go into how the weapon looks. Give it a name if it is a sword, or just a history. Have the character refer to it at times, and play up its importance to you. Even if your character is fine with upgrading to "magical rapier +4" for mechanical benefits, that doesn't mean you can't say the old sword didn't have sentimental value. It will also help bring the character to life.
As a GM with a duelist PC in your game, you're in luck. After all, what can be easier than challenging a duelist? They live for the duel. They want the fame, or at least the ability to test themselves out against others. So, all you really need to do is put another duelist around for them to fight. Build it up - if you want it to be important - and go from there. Perhaps put a duelist next to one of the big bads, or maybe just have them be their own thing. Just be careful that however you set it up, you give the PC the chance to face the person one on one. It's not a duel if the PCs friends all jump in to help, right?
Playing the Duelist
I've already made some big claims in this post about how a duelist is. Just look above in the "challenges" section and you'll see several. Now, your duelist does not need to conform to these expectations, but do try to understand what a duelist is. A duelist is someone who has chosen to focus on the one on one fight. They have trained themselves to be fast and accurate enough to dispatch a single foe with a lightning fast stroke, and to - in the case of swords - defend themselves against the same fate. This comes with a certain mentality that is different than someone who is just learning to fight. This is a person looking for that challenge, a pure challenge, of strength against strength. If that isn't your character's cup of tea, that is fine. They just may not be a duelist, and instead be simply 'duel capable'. Nothing wrong with that, but be aware of the difference, and let your GM know too. You don't want to be challenged on things that you don't actually possess, right?