Ah, another character type I have a real weakness for and therefore study with extreme suspicion because I don’t know if my liking it means that I’m more likely to excuse its flaws.
There should probably be some commas in that sentence somewhere.

1) The reason for being alone does not have to be angsty.

Apparently, some aliens invaded most fantasy worlds long ago and kidnapped or sterilized all the introverts. That would explain why 90% of the loner characters I’ve read about are the “Woe is me, my village was slaughtered/I am an exile/I am alone because of a curse/I left home because of unrequited love! Woe! Woe, woe, woe is me!” types.
Some people do enjoy solitude. Other people may lose connections and contacts with family and friends and lovers without there being anything particularly angsty about the loss. Others might leave home to pursue an ambition and wind up so immersed in their studies that they are not secretly pining for a lover. Some loners do choose it, some loners just get uncomfortable when someone else nosily pokes into their business, and some loners are not secretly so clingy that they’ll fall in love with the first person who looks at them sideways.
What reason for being alone makes sense for that person? I ask because I don’t find myself believing the angsty stories half the time. The author tells me this person was brave, self-confident, a lover of peace and quiet, and deeply settled into his home life—and then he became a wandering adventurer bent on vengeance and distrustful of all human contact when the villain killed his wife. Huh? What about the home he was supposedly rooted to, that love of peace and quiet, and that bravery and self-confidence? That had better be some damn convincing trauma if you really want me to believe that it changed every facet of the character’s personality. Much better to make his wife the reason he was rooted, the peace and quiet her influence on his temper, and the bravery and self-confidence springing from or linked to her in some way. Then I can buy that her death really would change him that much.
Not every loner’s story automatically has to be a happy one, but they also do not have to be emo.

2) Support networks, and lack thereof.

Most fantasy worlds have lower technology levels than ours, and magic is either uncommon or not used for practical low-level tasks like keeping warm, procuring food, and so on. This means that you should think about how, if your character’s a loner, he’ll stay alive. Community was such an important bond in the pre-modern world precisely because most people couldn’t survive on their own. So how does the wandering adventurer who travels beyond the walls and spends many nights in the forest on his own survive?
First of all, unless he’s a powerful mage himself and the magic is of the kind that requires no outside materials, I’m going to refuse to believe that it’s completely on his own. He’ll need to travel into villages to procure grain for his horse, to pick up things like new swords that he can’t make by himself, to get some food that isn’t what he could hunt or gather in the wild, to hear gossip, to sell or trade any goods he does make, and to talk with the “contacts” that the most die-hard “loners” still seem to have. There’s also the fact of simple human company, although there are the emo types who will start insisting that they can’t trust or talk to anyone ever because they are cursed oh noes. In times of war, when the soldiers rampage across the fields and forests killing and burning everything in their path, he’ll probably want to withdraw behind the nearest town’s walls for protection (even if he managed to keep the soldiers away from his house, he wouldn’t have anything to eat).
Second, there’s the fact that so many loner characters seem to have such a hostile relationship with the settled people nearest them that I can’t believe in it, either. Okay, so this outcast witch lives on the outskirts of town, and everyone believes that she has the power to kill them all in their beds and she’d do it for no reason other than pure evil. Sooner or later, the constant fear is going to mutate into hatred, and they’ll drive her away or kill her. And then there’s the so-superior hero who wanders through the village every spring and sneers at everyone when they stare at him in awe and refuses to pay for his food and drink. Perhaps none of the villagers has the skill to best him in a swordfight, but I would expect saliva in the ale, mocking stories to circulate about him, and the local equivalent of, “So sorry, but I’m washing my hair” when he tries to hook up with the barmaids. If there’s no reason to tolerate an asshole, no benefit that the villagers get from his presence, and a mutual hatred for them on the loner’s part, why should the village just cower before the asshole?
Third, there’s the fact that it would be interesting to write the story of a connection forming between a loner with high walls and a character intense enough to get through those walls. Yes, intense enough. Remember that not every loner has to be secretly clingy? That means that not every loner has to be the kind who’ll melt for a child dropped on the doorstep or the first pretty/handsome person of the preferred sex who comes to the door. I’d prefer intensity, thank you. And the connection could be friendship or companionship, not mutual love.

3) Romanticism has to be earned.

So you’ve got your character alone, and he is Tortured but has a good reason to be so, and you know how he survives. And now you’re ready to write the romance of how he falls in love with an equally solitary woman and they wind up alone together against the world, save it, and then gallop off into the sunset together.
Whoa there.
I admit you’ve got a good start. But just because a character is a loner does not make him/her automatically a good candidate for a romance. In fact, I would think it makes it more difficult, since you’ll have to get past whatever reason he/she has for being alone in the first place.
The reason this doesn’t often show up is the author’s reliance on “loner= sexy.” Sure, it’s built into a lot of our cultural stereotypes. But, though I might be fully in sympathy with an introvert hero who’s getting dragged into this crazy quest to save the world against his will, that doesn’t mean I’m going to think he’s good enough for the heroine if he emotionally abuses her and treats her like shit in his desire to be left alone. And if the heroine forgives him everything “because she understood him,” the book gets thumped closed and thrown away.
Your character is not a clockwork toy (and if you have ambitions in that direction, I hope to whatever deity you believe in that the character really is a clockwork toy or that you don’t mind stock types at all). Solitude might inspire sympathy. It might inspire romanticizing. It does not mean that all the loner has to do is stand there and have the hero/ine fall at his or her feet, or that the reader must sympathize because you’ve thought up a Tortured background. Please do not make your other characters act like doormats. Please do not make the whole point of the story a pity-fest for your loner protagonist. Please show what traits the loner character has that he/she could bring to a real romance or other type of connection.
I’m especially hard on this point because, as previously mentioned, I have a huge weakness for characters who act on their own, rebel against their destinies, or form a close connection with just one or two other people. I hate it if the author relies on just that, the same way I do when the author relies on beauty to get me to like a character. But I’m always afraid that I’ll get tempted to excuse it, because of my own fondness. So. Quit it.
(This has been your daily dose of too much looking into Limyaael’s brain).

4) Altered interior life, here we come!

This loner character has lived alone in the wild for ten years, with only occasional trips to the village when he really needs something he can’t make or grow for himself. His only companions are his horse and his hounds. His nearest neighbors are the large wolf pack that lives in the mountains. He might not speak two thousand words in a year, and this is just the way he likes it.
What does his mind look like?
There are a couple of choices here. Perhaps he has an extremely rich and complex interior life, given that he has to fall back on himself for entertainment. Perhaps he’s a great artist, or has made important magical discoveries that he hasn’t bothered to share with anyone else, because admiring them in private is enough. Perhaps his mind expands to provide him with all the stimulation and deep thoughts that company otherwise would.
Or perhaps he’s shrunken back into himself, with many parts of his social skills just gone. His trips into town are near-disasters, since he’s always acting weirdly. He thinks of little but food and practical tasks. His priorities have shifted, and if someone came seeking him to save the world, it would take him some time to get them straight again. It’s amazing how much we rely on other people to define us. If this loner has spent enormous amounts of time apart from society, he may not be as much of a person as he was before.
Or perhaps this is just one stop on a long, long life, especially if he’s immortal or non-human. Then he might be only a bit different from what he would be otherwise, but it would still have affected him. Choosing to spend ten years in the wild might be a perfectly rational step for an elf. On the other hand, will speaking with other elves just a few times a year really make him as charming and quick-witted and informed on current events as someone who lives in the elven village year-round?
I will now stop rambling and distill this down: If you write from his viewpoint, tell the story as if he’s telling it, and not just someone with a few nifty dark shadows in his past. Show the impact of the solitude. If it’s not from his viewpoint, then have the other characters notice something different. Otherwise, you’re once again aiming for a “cool” trait without wanting to portray the associated costs. And you’re giving up some damn cool stories, to boot.

5) The loner’s attitude towards solitude will differ depending on his/her conception of its purpose.

Someone hiding alone in a cabin because she thinks she’s cursed and fears hurting others when she transforms into a werebutterfly will be different than someone drifting from village to village because he hasn’t really got anything better to do. The first person will probably fight harder to preserve her solitude. On the other hand, the drifter might be apathetic and not the best person to trust with the safety of the world if the protagonist has a choice.
So, why is this character alone? I don’t mean the background events. Yes, she might have run away after her village was slaughtered. But what keeps her alone, rather than sending her into another village to pick up an adoptive family and new friends? What is the purpose of the solitude it her own mind? And if it’s the angst, then at least consider that someone brooding constantly on the events that made her a loner is probably not going to be the most pleasant person in the world, and it doesn’t guarantee that she has razor-sharp wit.
This is another place where authorial reasoning appears to fall into the gap. The author knows that she wants the character to become a loner because of an angsty reason, and keeping her a loner will work for the plot. But why she stays a loner once the initial angsty event is past is never explained.
So. Explain it. Fit it in with the character’s personality. Know how she sees the solitude. That will define a great many things, from more personality characteristics to how willingly she interacts with others.

6) Getting information to your character becomes more of a problem.

I’ve alluded to this in a few other points. It’s time that it had a point all of its own.
It’s possible that a character who lives alone in the middle of the woods will know that the village nearby has a new mayor whom everyone despises—though, unless his visit happens to coincide with the mayor’s ascent into office, he’s unlikely to know it right away. However, this fact will not necessarily be particularly significant to him. He lives out in the middle of the woods, no one ever visits, the new mayor is leaving him alone and has no known connection to his enemies. Why should he care? A loner character’s extreme startlement and suspicion on learning a new fact that the author intends to turn into a plot hook would best hinge on the fact affecting his own life.
Even more damaging, a loner character will not pick up on larger patterns of facts that might indicate to someone in the know that Something Wicked This Way Comes. Perhaps every village in two hundred miles has a new, despicable mayor, and there are rumors of the Dark Lord’s return, and people are starting to put two and two together. Yet is anyone going to hotfoot it out to the loner’s cottage to tell him every rumor? More to the point, would he necessarily be friends with the one person who just happens to figure out that SWTWC? He might be intelligent enough to figure it out, but in the absence of knowledge, wisdom does little.
There are ways to overcome these problems, of course. Here are some I’ve seen authors use when trying to get information to loner characters:

  • Pros: Don’t generally have to explain it, an accepted literary convention.
  • Cons: Some readers will notice the lack of explanation, turns language fuzzy (once again, the dreaded “something told him not to say anything” or “somehow he just knew.”)
Secret contacts
  • Pros: Logical, gives the loner character a connection to society and people to care about.
  • Cons: Needs to be a logical reason for these people to become contacts in the first place (can the loner actually pay them, for example?), needs to be a logical reason for the loner to care about such rumors or information.
  • Pros: Acceptable in a fantasy context, rarely needs to be justified.
  • Cons: Very cheesy, extremely poorly-used much of the time.
Messages from afar
  • Pros: Comes from a source close to the epicenter of events, is exciting and dramatic, often pulls the loner back into contact for emotional reasons.
  • Cons: Logic problems (how does the informant know where to find the loner if he’s covered his tracks so well?), almost invariably a sign of an angsty past.
Outsiders come and fetch him
  • Pros: Convenient, can bring up the reluctance to engage in heroism that so many authors love, demonstrates the seriousness of the world-shaking events, clear and accurate information.
  • Cons: Overused, loner character often changes dramatically overnight or begins to angst, the reluctance becomes a cliché.

7) A touch of ordinariness can work marvels.

Most fantasy loners tend to the extreme—as mentioned above, 90% of the time it’s the angsty extreme—and even if they chose their solitude, tend to have done so after a heroic fight in which they proved how goddamn cool they were. When they get dragged back into saving the world, of course they show how wonderful they are once more, and end up with either a wonderful romance or a chosen solitude that is presented with awe and worship.
And yet I’m sure most of us know people who spend nearly all their time alone but are largely ordinary.
Just as the character’s solitude isn’t an excuse for Torment or having the love interest fall in love all by itself, neither does it have to indicate larger-than-life angst or power. If it tends in that direction with no temperance, you’re a lot more likely to tumble into stock types.
Luckily, you can get around that. You can show the consequences of solitude on the character’s psychology, as I’ve mentioned in the other points. You can show that the character still has flaws that don’t relate back to the solitude. You can show him or her having many different kinds of bonds with people, not just icy indifference and frantic friendship or love. You can show him or her making mistakes and getting humiliated, rightly, over them. You can show the character facing whatever problems made him or her retreat into isolation in the first place.
It’s wonderful what can be done, when you start thinking of them as solitary people, not just loners. Remember: Not all loners are lonely.
Next we get character change as gradual process. Oooh.
*chokes temptation to write rant one day early*