A Mary Sue is a 'perfect' character. The term originates from fanfiction. The fan author inserts a female (*) character who gets all the cool boys, is the most beautiful, the smartest, the... you name it. If anybody is her enemy, it's because they are jealous. In short, the character lives all the wishes and dreams of the author.
Eg. a Mary Sue in a Harry Potter fanfiction can convert Voldemort to the good side because she's just so beautiful and innocent etc...
Here's the Wikipedia page: Mary Sue - Wikipedia, the free encyclopedia
(*) The male version is called a Gary Stu.
What type would a Mary Sue be? Or can they be of any type?
Or does it depend on the story - eg. an E/I-NFP in a fantasy world and a E/I-NTP in a science fiction world?
I'm betting on N because - just because of that "ideal" image of N. A Mary Sue has a deeper understanding of her environment than anybody else. Unlike her, real human Intuitors aren't always right with their 'understanding'.
I'm also leaning towards P... Of course, again, this is not the real Perceiver who is too late for classes and gets berated. This is the one who's too late for classes and gets berated, but turns out to be RIGHT because there WAS something more important to do.