A Mary Sue- or a Marty Stu- for that matter, is an overly perfect character. These types of characters are usually talented to the extreme and are loved by every single person on the planet. 

Hey everyone, my name's Mary Sue! I have luscious golden locks that reach my waist, a slim figure, tan skin, aqua blue eyes, and unnaturally shiny teeth. I'm also so super nice and I can play the piano, juggle elephants, play soccer, football, baseball, and tennis, and burp the Star Spangled Banner.

Congratulations, would you like a cookie?

I also love people, and everybody loves me. I don't have any freckles, nor a scar. My face has no blemishes. I have never had acne and I am pretty much bald except for the hair on my head. 

*slowly claps* 

Now, how many of you have ever met someone as perfect as this? No one? YES, because nobody can be as perfect as a Mary Sue. Truth be told, she's just too perfect and too unrealistic. Now if you guys want to create a good character, you have to give them flaws- just like everyone else in the world. Plus, the reader probably has a deep hatred for this Mary Sure character deep down in their hearts.

>.< 

Now, a 'Mart Stu' or a 'Gary Stu' is basically just the male version of a 'Mary Sue'. He's overly handsome, doesn't have any flaws, and is super athletic and stuff...

Then there are the other ones... let's call them 'Doesabsolutelynothing Sue' and 'Theboyversionofdoesabsolutelynothing Sue'. 

As their names might imply, they do absolutely nothing. However, the whole world basically revolves around them or they think the world revolves around them. 

*facepalm*

What is this? If you want people to do something for you, you have to do something for them!

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