In fiction, what would you say would make a character a Mary-Sue or Gary-Stu (I'll just use the term Sue to make it easier)?

(Note that this contains some spoilers for various TV shows.)

In Superhero types of stories, many characters seem to qualify. Before I continue, I just want to say that I'm not trying to insult anyone or any character. I'm just curious.

The one main rule that I have heard that makes a character a Sue is stealing the spotlight.

A character can do this in a few different ways. The main one seems to be a character stealing the role of another character, especially in relationships.

However, with that example, Bruce Wayne in World's Finest establishing a relationship with Louis Lane technically was Sue'ish behaviour. Or was it? Some say that if an official character does it, it's all right. It's only when original characters do it that we have a problem.

Another example is over powered characters. However, some might say this is an example of "God-Modding" rather then being a Sue.

Some might call Superman Godmoddy, but does that make him a Sue? From what I've seen, the answer seems to be no.
However, if he were to guest in Batman (and I'm talking pre Justice League, so when we wouldn’t expect to see the two together) and took out Batman villains easily, (essentially stealing the spotlight) would that be sue-ish behaviour?

Again, from what I’ve seen, overly powered or intelligent (i.e., characters who are smart to be point of absurdity) heroes don’t seem to be sues as much as they are simply over powered. I think they only become sues when their abilities start stealing the spotlight from other characters.

An example of that could be Wesley Crusher. Being a child genius in itself is not necessarily sue-ish. Allowing him to always solve the problem to the point where other characters in an ensemble cast take the back seat to him is IMO sue-ish.

Another example of that could be brining in a new character. All of the sudden, said character becomes of focus of the series to point where you might as well call it their show staring some other people. Just look at how many people started calling Voyager “The Seven of Nine show” after her character came aboard.

In the end, the main rule seems to be, not necessarily having a character being powerful. However, stealing the spotlight or role from another character is another matter. So, if one has an original character in a fanfic and said character has a relationship with an established character, more or less stealing the role from another established character that normally goes with the character, that is sueish behaviour. If an original character is so smart and/or strong that they’re constantly accomplishing things established characters do, then that too can be sueish behaviour.

Anyway, I’m just wonder how others feel on this issue. Are there other things that would make a character a sue? What other examples from fiction would quality as sueish behaviour?

I feel that a Mary-Sue is a character who can be described as "they're like me only better." The idea is that the author is living out their fantasies and dreams through the character. In that way the author is like Barkley in TNG when he created those holodeck fantasies where got to be a badass who stood up to, and bested, everyone around him and, of course, got the girl.

I saw a great list of signs that your character could be a Mary-Sue when making fan-fiction. It says that the basic thing with Mary-Sues is that they are a kind of wish fulfillment for the author by putting them into their favorite TV-Show/Movie doing all of the thing that they have fantasized about doing. Among them were:

Does the character share hobbies with you, or hobbies that you wish you had.

Is your character a master of some skill that you wish that you had (i.e. master computer hacker, master of some martial arts)

Is the character named after you?

Does your character constantly show up the previously established characters at everything they do.

Does the character go around being a jerk towards others and yet is universally loved by them. This is part of the wish fulfillment idea, saying that the author feels that they are constantly pushed around and stepped on by others and those others always get away with it. Sort of like on TNG when Barkley uses the holodeck to cast himself in situations where he tell off superior officers, beats them in sword fighting and gets the girl.

On the flip side, is your character misunderstood or unfairly persecuted?

Does your character have supernatural powers like ESP or telekinesis, epically in a universe where such powers have never been established to exist before.

Does you character have a tragic past but no signs of having been traumatized by that tragic background?

Does your character have some exotic appearance that seems unique or out of place in this universe.

Is your character so attractive that it stands out and is often commented on and described in great detail.

Is your character a hybrid of races who we've never seen produce offspring together before.

Does your character get romantically involved with a canon character.

Is your character related to a previously established character (i.e. long lost brother/daughter/son)

Has the character accomplished things that someone their age shouldn't have been able to accomplish yet (i.e. the youngest captain in the fleet, 18 year old PHD)

Does the character dress in an unusually sexy or unique manor that doesn't fit the universe or place of the character in that universe. For example a female character who wears a leather midriff baring, cleavage revealing top, leather micro skirt and thigh-high boots in a Xena story would be expected in that world but as an FBI agent in an X-Files story wouldn't be.