Alright, I just took the test. I have to admit, that my first Vampire character Nick, was some kind of Mary Sue at the start, But as time went on, and I had gotten to play him I developed him more and the final result was 14% Mary Sue on that test. This is what the test said about the Charater: 11-20 points: The Non-Sue. Your character is a well-developed, balanced person, and is almost certainly not a Mary Sue. Congratulations!
To create a balanced character I think the best thing to do, is to start with the Virtue and Vice, and really sink your mind into them. You need a balance of what is good about the character and what is bad. Same thing when you are working on his/hers backstory. There need to be an even blend of negativie and positive experiences in her childhood -- And young adult life. Personally I think there has to be more negativities than positivities... Be it experiences or deeds that she/he has done, that does not matter. Most of us dwell on what is negative, so what would the character think of as negative, what is her "angst" What is her fears, what is her regrets, and believe me, she needs more than one thing to be afraid of, angst, or even regret. This is only one of many things that needs a blance ofcourse. The reason I think there should be more negativities than possitivities is that we don't sit down and play a game, where everyone is happy, world peace and that shit. In such a universe there would be nothing to drive the story, no complications, no drama. The negativities create drama. And a chracter with a dramatic ground helps create further story.
So, basically Vice, backstory and personality is the three things you have to look at if you don't want to create a Mary Sue. This is all none stat things, and might actually do the whole job. But stat vise, the single most important thing to remember is that - "If you think you are the shit, I'll show you one of theese and we'll setlle the matter for good" - As a very nifty Gangrel once said. There is always something worse than you out there, and you damn well better not forget that.
We all, as every good fictional characters has flaws, be it mental or physical. One person might have a sweet spot for ladies, making him unfaithfull in that regard, and so on and so forth. You MUST add things about the character that is questionable. Even saint's has flaws, be it something as simple as beeing overly righteous which blinds him and leads to all sorts of complications.
In fear of repeating myself over and over again, I'll stop now. I really hope some of this helped....
Did it help some?