Disclaimer: I don't own X-Men.
Disclaimer--the Sequel: I am not an expert on Mary Sues (or, for that matter, writing), nor do I claim to be one. This list was written primarily for my own amusement.
Original characters have shown themselves to be rather popular for writers of X-Men: Evolution fanfiction. However, because said characters are the author's personal creation, and thus have a special 'bond' with their creator, it is tempting for authors to make their character exceedingly perfect, with an amazing array of powers. This guide lists some of the warning signs which are clues that your character may, in fact, be a Mary Sue.
You might need to rethink your character if:
A. They have the abilities of an existing canon character... but better.
Example: Molly could shoot beams out of her eyes, like Cyclops, but she was in complete control of her powers, so she didn't need to wear glasses. She could also control the path of the beams, bending them to her will.
Adding an OC like this makes the character they are 'based' off of pretty much useless. After all, there's someone else with all of their powers and more!
This might work, however, if you add greater repercussions for power usage. But that's a pretty big might, as the repercussions might lead to further angsting, which is one of the hallmarks of a Mary Sue.
B. They have the abilities of two or more existing characters.
Example: George had super-speed, could shoot spikes out of his body, and was pyrokinetic to boot.
Pretty much the same issues as above.
C. They have way, way too many powers.
Example: Marietta was pyrokinetic, cryokinetic, telepathic, clairvoyant, telekinetic, a shapeshifter, could create volatile balls of energy, could spontaneously sprout wings, could create tornadoes, absorb energy from other living creatures from afar, speak to animals, cause magical pots of tea to suddenly appear...
This is an extreme version of the above sign. Seriously, can anyone have that many powers?
D. Their powers make them effectively invincible/omnipotent.
Example: Keith had the ability to imitate the powers of any mutant in the vicinity. He also had an extreme healing factor-- mortal wounds would heal in mere seconds!
...Need I say more?
E. They have unusual physical features for no apparent reason.
Example: Zita was a telepath with large, expressive, purple eyes and a foxtail.
As far as I know, purple is not an eye color found in nature, nor is there any good reason for Zita to have the tail of a fox. She is a telepath. Foxes have nothing to do with her powers.
Now, if she were, say, like Rayne/Wolfsbane and could turn herself into a fox, the foxtail would be acceptable.
And perhaps she's wearing purple contact lenses as well.
F. They make canon characters act monstrously OOC.
Example: "You're just saying that because you are jealous," Rhonda said cooly.
"Oh yeah?" Kitty shouted. "Well you're a of a !" She stormed off.
"Don't worry, Rhonda," Lance said comfortingly, placing a hand on her shoulder. "Kitty's just a brat. Come on, sweetheart."
First off, Kitty doesn't cuss. Secondly, Lance is in love with Kitty. Even if the relationship were broken off, I don't think Lance would refer to her as a 'brat' (unless they broke up on really bad terms).
Making characters OOC for the purpose of your story is not a good idea, as it will tick people off, as well as causing the 'credibility' of your story to decrease dramatically.
Of course, if you have a legitimate reason for making the characters act that way (such as Mesmero messing with their heads) then that might be acceptable.
G. They make canon characters spontaneously fall in love with them.
Example: Albert stepped into the Institute. "Wow, he's hot," Kitty murmured.
"Yeah, he's definitely superior to Scott," Jean agreed.
"You said it," Rogue said.
If you're going to make other characters fall in love with your own, you should probably stick with just one or two. Otherwise, watching the females (or males, depending on your OC's gender) of the X-Men falling over themselves and getting into massive fights over your OC seems more exasperating than amusing. Unless one of your character's powers is the ability to cause others to automatically fall in love with them...
Furthermore, before breaking up relationships that have been established in canon (such as Jean and Scott), it would be best if you came up with a good reason for it.
H. Their past is angstier than Wolverine's.
Example: William had been abandoned when he was 1 ½ years old to fend for himself on the rough streets of New York. He had managed to survive, living on his own, until he was seven, when he was taken in by the foster care system. He was given to foster parents who hated him, making him do all the housework and locking him in a closet whenever he wasn't doing any chores. They ended up selling him as an experimental subject to a secret laboratory doing illegal experimentation. He had been injected with various chemicals which had caused his mutant abilities to manifest much earlier than they would have otherwise, and he trashed the laboratory with his newfound telekinetic powers, killing all of the scientists and escaping just in time to celebrate his eighth birthday.
Now, I know that X-23 and Wolverine both have rather angsty pasts. However, since they are canon characters, they are allowed to be such. However, original characters with equally angsty (or more angsty) pasts will be met with much less acceptance.
Hopefully, this guide has been helpful to you in the quest to create non-Mary Sue (or Gary Stu) characters. Happy writing!