I said that this Fanfiction stable will get its own chapter. Here it is.

It is well known that Mary Sues are beloved by all characters. Any characters that are against her are often villains.

They tend to be extremely powerful, without any training.
They have no flaws, well, not really. Some Sues have flaws, but these flaws have no actual impact. These flaws aren't a hinderance, no weakness the Villain can take advantage of.

They have nothing to overcome. Which means the plot is blant. No struggle, nothing to worry about. Boring.

Honestly, it is easier to say how to avoid the Sue than to define it. But before I do that, I would like to mention that OC's and self inserts are more likely to be called a Sue.

Relationships: Let your character be disliked, hated even. Not everyone will adore them as soon as they meet. And not everyone who dislikes or disagrees with them is evil. Maybe the allies have a good reason to disagree with the character.

Actions: Apropos, let the character make mistakes, let them be wrong. Let them fail. Let then not have every ability, skill or power in existence, let them be idiots about some topics. Let them feel negative consequences.

Abilities: Like I said, they don't need every ability available. But also, let them work for it. Show how a character learns archery, and can't hit shit for the first few tries.

Character traits: Let them be assholes. Let them have traits that creates situations, traits that make things more difficult. Let them struggle, let them feel consequences, and let these consequences have impact.