First things first, what is a Mary Sue?
Well, a Mary Sue is an original female character in fan fiction that serves as a somewhat obvious stand-in for the author, so that they can do what the author only wishes they could do and be what they wish for them to be, think of it as wish fulfillment.
She is the ideal girl. Whether she be an exotic, kickass, beauty with a dark past, or a sunny, pure, driven snow, beauty with a tragic past, she is the center of attention and the object of all desire.
Having a dark past or being beautiful in and of itself is not bad, but if it rules the story and the cast, then it becomes a problem.
Mary is a gorgeous bombshell whose description takes up half the story. Cool name, unusual eyes, and has long, flowing tresses with ten different naturally rainbow-colored highlights. And don't expect her to have a regular sized chest, oh no, they need to bulge like water melons.
She also punches people when they look at them, even if she wears a low-cut bikini top and a skirt so short, it would make a hooker blush, don't expect her to cut you slack.
If she has a devil fruit power, chances are that she possess the power to control water, yes the legendary, Mizu Mizu no mi.
If she doesn't have a power, she's probably a) an exceptional fighter b) an exceptional swordswoman c) charmingly beautiful or d) all of the above.
Her flaws consist of being clumsy in the presence of her love interest, being too self sacrificing, stubborn, and a boy magnet. If you expect these flaws to hinder her in any way during the course of the story, then you'll be severely disappointed.
Upon meeting, the crew will immediately accept her as one of them, except for Nami of course; someone needs to constantly look bad by comparison.
Whichever man is unlucky enough to be under the author's adoring eye will be immediately shunted into the role of love interest and be unconditionally, inconceivably, and irrevocably in love with her.
The other members of the crew are rendered useless in their love "plot" and either fade into obscurity or become their cheerleaders of love.
With that said, ON WITH THE TOUR!
Note: Any resemblances to anyone's actual OCs may be either completely coincidental, or completely intentional, the world may never know.