It depends on a lot of things.
The biggest thing to consider is importance to the story. If your OC is the protagonist, there should be a reason for it beyond "I wanted to make my OC the main character." What traits does your OC have that make it so the canon main characters couldn't tell this story? For example, I wrote *
a Harry Potter
fic with a Slytherin boy as the main character. It had to be an OC, because it was an exploration of the inner turmoil his family's prejudice created in him; I couldn't have told this story with Draco or the Trio. Ask yourself if you really need that OC, and if you decide you do, move on.
Second, since you're playing in an established universe, consider where this character comes from in that world. If the OC is going to be a main character associating with established mains, you need to include some explanation for why they're there. For example, if it was a Harry Potter
fic and your character interacts with the Trio, you should give a logical explanation for why we never saw here before (e.g., she could have been Beyond The Black
or in another House). This is why American Transfer Students fics get such bad press— the reason the student shows up is cheap and stretches credibility.
Third, as with any character, make your OC real.
1) Give him a personality. Try the "two good traits one bad trait" method of creation. Like, say, Smart (good), Methodical (good), Arrogant (bad). The two good qualities prop up the bad one— because he's smart and does everything painstakingly well, he lords it over everyone else, making him insufferable. The good traits can be pressed into bad one— he's so methodical, he takes too much time correcting his essay and doesn't hand it in in time.
2) Attach traits that make sense for his personality. Is he a picky eater? Does he hate being tickled? The more traits you add, the more lifelike he becomes.
Ideally, you should try to fit traits together that make sense. Our arrogant smart guy could be fussy about his eating habits— he only puts one thing on a plate because he hates to see food touching. He eats slowly because he's methodical— by the time he's done the salad, everyone else is done the meal. It wouldn't quite make sense for him to scarf down his meal; it would go against the personality we set down (unless you want him to be methodical when working, but slovenly elsewhere. You can experiment).
3) Give him a history. He had to exist before the story started— what was that like? Who were his parents? How did they affect his life? Did he do well in school? Did he have friends? Why or why not? What was his best memory before the story started? What about the worst? Something that made him ashamed? What was he doing a year before the story?
Making a character "real" goes a long way toward making them believable and likable.
Finally, I would suggest cutting down on tragedy. Tragedies are often foisted on characters as a cheap way of making them interesting, but it's usually not and tragic backstories quickly get out of hand. "My parents died and my foster parents beat me and they suppressed my magic that only manifests itself when I cut myself and the only boy I ever loved is a vampire who loves someone else waaaaaaah."
One or two of those, sufficiently explored, will suffice. Back to our Smart Guy. His parents died and he was sent to a foster home. They didn't beat him, but they didn't really pay attention to him. Building on what we know of his personality, Smart Guy thinks that if he works hard, they'll notice him, so he takes great pains to do all his work perfectly. He does things slowly and carefully so he doesn't annoy his foster parents. He acts arrogant and always announces when he does things well because he wants people to notice him.
His tragedy influences his actions, building on and exploring his character. The tragedy isn't overwhelming— it just makes him sympathetic. He's still arrogant, smart, and methodical, but he has an added dimension now.
And that's my fourteen cents. Here's a good source for this stuff.
I like that site; it has lots of practical information.
edited 9th Sep '10 3:54:15 PM by Ronka87