Quick anti Mary Sue guide
Part I: The test
Pick up a piece of paper. Now pick up the character that you find susceptible of being a Marty Stu/Mary Sue (for the sake of convenience, let's call him/her Character M.).
Step 1: Pick up three different characters. For each character (A, B and C, respectively), write down what he or she thinks of M.
Points you might have to take into consideration:
what the relationship between the characters is;
how the characters met and what their very first impression of each other was;
the overall vision that the characters have of M, based on
a) the interaction between them;
b) how characters A, B and C perceive M's behavior and actions in general and the relationship between M and other characters;
c) what A, B and C know of M, either from M him/herself, from deduction or from another source.
Step 2: Write down what M thinks of him/herself.
does he or she have a very clear idea of their very own existence in their very own universe?
is he or she strong enough not to depend on anything/anyone else?
does he or she realize their very own limits and values?
Step 3: Write down what you as the author think of M.
why did you feel the need to create him/her in the very first place?
is he/she a reflection of what you are, what you once were or how you'd like to be?
do you view his or her qualities and flaws with awe, respect, envy, pride or disgust, contempt and malice?
is he/she deliberately based on/inspired by anyone in real life or did you accidentally create him/her similar to someone to whom you have a distinctive relationship?
Step 4: Check your results.
Your character is most likely NOT a Mary Sue if either of the following is true:
from the six opinions written above, at least three are different;
there are at least two negative opinions.
Part II: Explanations
Most writers that are new to creating characters have the wrong notion that a character is made up of likes and dislikes, abilities and inabilities, habits and distinctive characteristics in appearance. In order to reinforce the idea that their character is new, refreshing and interesting, they try to add as many traits to the above listed categories as possible. One common mistake, for example, is to operate on the character's manner or speech without considering their background and why they speak so in the first place this includes adding a pointless accent or an anime style verbal twitch ("Nyaa!").
However, the features included in the mentioned classes are only manifestations of the essence of the characters and not the essence itself.
A character, like a real person, is made up of two major factors:
1)The outward perception the part of the character/person that is visible to everyone at anytime. This is either restricted by the character/person themselves or comes up involuntarily in critical moments and is a great way of characterization.
2)The inner world of a character/person what the character/person actually is, provided that they stay true to themselves. This includes not only consciousness, thinking processes, desires, goals and evolution of feelings, but also the value scale, which can be either implemented by someone else or created by that very person.
Of course, it you want to apply this more thoroughly throughout fiction, there's also the meta factor:
3)How the author (God) who knows everything plans to make that character develop and reach their goal/ advance the plot/ prove a point.
Part III: Defying the common misconceptions
A green-haired, 500-year-old, prophecy telling powerful half-human, half-elf warrior called Cassandra doesn't necessarily have to be a Mary Sue. Sure, she most probably is, but the mentioned traits don't automatically make her one. In fact, she can actually become a very realistic and well-developed character, provided that:
the Mary Sue like characteristics are not the central part of what makes her a character. This can be avoided through various means. For example, the tone of the story can be a very mature and serious one, making her have a very realistic manner of perceiving the world that eclipses her almighty nature; or she may decide to impose on herself a certain limit as to what abilities she may use and when.
she has a conflict worthy of her powers. Either her enemies are as strong as she is and the battles are very tight making her lose in the same percentage she wins or her inner conflict, defined either by her flaws or an important dilemma, requires all her resources to be fought against. Remember that a certain equilibrium between the two representatives of a concept should be attained, otherwise the story isn't cut well enough to draw attention from the readers.
the character actually goes through some effort and doesn't have her Mary Sue traits for granted. There's always room for developing a character; however, most often presenting the character's background is thought of as being easier to perform for a writer. Unfortunately, this can also lead to the other extreme a character that doesn't have anything else besides a background (usually found in tragic Sues) and no goal of their own or purpose for the story whatsoever.
Part IV: Conclusion
There are a few points that need to be restated from this guide:
1)It is very unlikely that two characters have the very same perception of another one, as it is unlikely that two real life people think the very same things about the third one. Opinion is formed mostly through experience (which in turn is formed by taking part in the same situation) and filtering thoughts through one's own system of values (which is unique to each and every one of us).
2)Don't be afraid to come up with and write a character whom you'd hate, had him or her existed in real life. A lot of published authors admit that they love to write their negative characters and a lot of readers admit they love to hate them. Be creative you would be surprised yourself to realize what you're capable of inventing outside your usual tastes.
3)Don't indulge yourself in the guilty pleasure of writing a character you love. In fact, don't be afraid to see your character from an objective point of view and dislike him or her because they're so easy to write. Make sure you are the one controlling your character and not the other way around.
4)There's one thing to loosely base your character on someone you know and an entirely different story to make an exact fictionalized copy of that person. As much as you can, try to avoid the latter.
Thank you for reading this far I hope this guide proved to be useful. And don't forget: be open to criticism. Your characters' flaws may be impossible for you to spot but evident to your readers.