Like I can’t stress this enough, there’s no definition, I’ve literally seen the term be used in so many different ways that someone like Saitama or Guts could be considered a Mary Sue. Don’t say they can’t be because whatever definition you have can be countered by a different definition. The problem here is that neither of you are wrong technically.
So what I’ve learned from my experiences are:
1.Anyone’s a Mary Sue if you try hard enough
2.Everyone isn’t a Mary Sue if you try hard enough
3.This term is used to try and ruin a reputation of a character in the eyes of viewers
4.This term is used by people who don’t really think about it and wants a catch all term
5.The term is used by lazy people
I believe most people are 4.