Majority of Mary-Sues are orphans or have dead parents. (I think there's a difference?) They then go on to have to survive alone and therefore become badass warriors or some other sort of fighter.
I notice DC (and probably Marvel) comics do this a lot which means they might partially be to blame. Batman (aka Bruce Wayne) and Superman (aka Clark Kent) are both orphans who have made it big as superheroes. (Don't shoot me DC fans, I know there's more facts but please, I only have 250 words.)
Back to my point. Many of these Mary-Sue characters then go on to do something unrealistically awesome. The amount of these that become international spies is breath taking. And unlike Black Widow (aka Natasha Romanov), who suffered horribly, these guys have had it relatively easy.
What makes an orphan or someone who's parents have died a Mary-Sue, is how much they've achieved in comparison to how much they've suffered. Someone who is heavily mentally scarred but achieved much or a character who achieved very little but easily is much less of a Mary-Sue than that character who suddenly became a ninja at the age of 16 in a single montage. What I mean by very little is maybe they're still a rookie spy past the age of 18 (so when I say little or a lot, I'm referring to a career as a secret agent).
I really hope I haven't offended anyone in this chapter. I'm really sorry if I have X.
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Anti Mary-Sue Character Guide
Non-FictionHere is just my list of tips of how to avoid creating/writing a Mary-Sue character. Take them as you wish. Greatest Achievements: #1 in non-fiction!!!