Primary trait of any Mary Sue character is that Mary Sue characters don’t follow the logic of the world they inhabit, instead they either warp the world logic to their favor or just follow it very loosely so having a MS character is never a good thing. One of the primary derivatives is when character starts pulling new powers, abilities, items or information out of nowhere with no valid prerequisites precisely at the right moment. The same thing goes for pretty much everything else:
– Character is loved by everyone without any valid reasons
– Character has an ability that literally no one else have and doesn’t have any valid explanations for it
– Character don’t have flaws that it should according to world logic
– Character wears an apparel that makes absolutely no sense or, as I like to call it, “bullshit fantasy armor syndrome”, and never suffers any consequences
Another important point is about being “overpowered” – this is not an entirely correct term because the right description would be “unrealistically powerful according to world logic”. This becomes painfully obvious when your character can overcome many enemies without any reasonable problems, especially when said enemies start acting like brain-dead morons when character happens nearby. That particular trait of Mary Sue was one of the main reasons why I’ve started my own comic projects because I wanted to show that if you take a typical fantasy “hero” and pose it against an enemy who is actually competent, favors will be strongly on the side of an enemy and not the “hero”.