I see, but what about canon characters? People have done those things to them to make them Suish/Stuish.What is your favorite genre to read?
Fantasy, for the most part, though I'm primarily interested in fantasy novels that break the traditional "hero goes on an epic quest to defeat dark power, epic cycle" model. I also dabble a bit in post-apocalyptic sci-fi, and lately I've been reading a lot of 19th century lit, like Dickens, Hugo, etc. As far as fanfiction goes... I actually don't read fanfiction.
Do you use Original Characters in your work?
Seeing as I'm working strictly in original fiction right now, yeah, I do. And even when I dabble in fanfic, I use OCs. Back in Hero's Path, a few canon characters made cameos, but the focus was mostly on the characters I had personally developed. It's a matter of personal preference really, but I feel like I can develop a better psychosis for a character I've built from the ground up. However, I think I'll be working on a Nuzlocke fic soon about my run through Diamond, and by its nature I'll have to use a lot of the characters that appear in the games. I'll still keep them in-character, obviously, but I'd like to add my own spin to it. Like, for example, I think I'm going to make the rival a cancer patient who has gone through chemo, and he's racing through his journey because he wants to finish before he has a relapse.
What defines a Sue character?
Well, TV Tropes gives a pretty good definition and a [http://tvtropes.org/pmwiki/pmwiki.ph...SueTraits]list of common characteristics[/url], which should probably answer any questions you have. If you want a more personal definition, how I specificially would define a Mary Sue, it's an original character that the writer has gone out of the way to make perfect, or at least tries to make the audience perceive as perfect. They don't have any flaws (or very few flaws) within the story, but they are most certainly are spotlight stealers. They warp the plot around them, too. Now, there's a difference between being central to the plot and warping it. You can have a character who figures largely into the plot and causes many plot events to happen around them. Protagonists, by nature, do this. But it's a different matter entirely when you make other characters act counter to their characterization just so this other Sue character can accomplish something.
I will preface my next remark by saying I have not read Twilight. I have seen the first movie, and so if there is a difference in characterization, then I am focusing solely on the character of Bella in movie adaptation. See, she's someone who strikes me as a massive Sue because she's the new girl in school, with a fancy, flowery name and a very vanilla characterization. She seems like a very bland person, and yet everyone is bending over backwards for her, laughing at her poor attempts at humor and generally fawning over her. See, that doesn't happen to the new girl at school. No one pays attention to the new kid. They most certainly do not fawn adoringly over the new kid. And yet she warps the natural inclinations of characters so the story can do what the author wants it.
Basically, a Sue character is a violation of one of Vonegut's storytelling techniques, be a sadist. Nothing bad happens to them, or if it does, it's not permanent. I remember in the Eragon books, the titular character receives a scar on his back that somehow prevents him from wielding magic and also hampers his ability to fight. This was a cool plot element, making him have this whole psychosocial struggle. But not too long after, not only is the scar healed and his status as a cripple removed, but he also gains increased stamina, speed, and strength in the process. His handicap is absolved, and he's given new powers for free.
So a Sue is basically a character the author has projected themselves onto rather strongly, and does not want to see bad things happen to aforementioned character. They manipulate the world of the story in ways that don't make sense to the readership just so they can make their chosen character shine. Basically, just read the TV Tropes pages, it's got everything I said, put a little more eloquently.