What do you mean by "Mary Sue"? I've seen that term thrown around and have to assume what they mean.
The original meaning was a character written into fanfic that represented what the writer wanted to act out within that area of fiction. The writer effectively inserts themself into the world. Originally for Star Trek fanfic, the 'Mary Sue' is written as being prominent and admired within that setting, and usually manages to save the day and/or sleep with the writer's love interests.
In RPGs, a 'Mary Sue' is an NPC created as a way for the GM (or game writer) to insert themself into the gameworld and be a part of it. 'Mary Sue' NPCs generally end up more powerful or skilled than everyone else, including the PCs, and are a pain in the rear end when encountered by players because they usually act like Über-PCs run by the GM.
https://en.wikipedia.org/wiki/Mary_...n%20written%20by%20adolescent%20authors.
