A writer will give her/himself away in the attributes of their character. Often you see or understand the author's own inner-being through one of their characters. We must be careful not to make all of our characters reflections or interpretations of ourselves, or every story will sound as if the author is living in each individual story. If you have loyal readers this can work against you. Your reader will begin to hear you in every story and stop reading your work.
Worse than that...if you're making your character a reflection of you, only an ideal you. Your character becomes a 'Mary Sue'. Too perfect, too one dimensional and too unrealistic.
Here's a short list of things to watch for if you think your character may be a 'Mary Sue or 'Tommy Too Good'
1. This character is uncommonly beautiful/handsome.
2. This character is very smart.
3. Other characters often feel compassion towards this character.
4. The majority of the other characters like this character.
5. This character has some/many physical or psychological characteristics that can also be found on the writer or on the person the writer aspires to be.
6. This character learns new skills rapidly, often being labeled as “a natural”.
7. This characters’ physical or psychological characteristics allow them to be labeled as perfect.
Even if your character is imperfect and shares your own flaws and weaknesses but overcomes them in seemingly inhuman ways, you've got a 'Mary Sue'.
Ask yourself these questions-
Does your character share physical attributes with you?
Does your character share a background with you?
Does your character have insurmountable odds that are solved quickly?
Does he/she possess other-worldly powers that make him/her safe from everyone else?
Does everyone like him/her? Even the bad guys?
Does your character have a physical trait that makes them unattractive? mental or emotional trait? both?
Is he/she intelligent and/or witty? both?
Take the answers to these questions and flaw your character. give then a speech impediment or disease that makes them weak. Let the other characters be frustrated, impatient, disgusted and/or disappointed in your characters choices. Do Not have someone else in the story convince the character its not their fault. Give them something that is their fault and make them live with the knowledge they're not perfect.
These have helped me in crafting both male and female protagonists as well as a test you can give your character I found on-line. Google search-Mary Sue test and run your characters through the gauntlet. One of my favorite characters is a Wizard who is always a mess, too tall and awkward and gets himself into trouble in which he gets the crap beat out of him and/or blows something up. He's fun, funny, real and relateable because he works so hard to be special. Give it a try and keep writing!