I think what really makes a Mary Sue is a combination of over the top attention, favoritism, and crappy writing. Your character isn't a Sue because (s)he had a bad past; it's when they whine about it all the time, randomly mention it, or it has nothing to do with the plot, or all of the above. Don't you agree?

I'm curious, how is the Mary Sue on the main page's outfit revealing?

That first mentioned litmus test is horrible. Seriously. Any character that's unusually interesting is a sue, according to it.

"she's probably the one telling the Sorting Hat which house she's going to be in." Was this choosing one's own house not specifically presented as an option students may select, as part of the story's free-will theme?

"The same character tends to appear in all of the works by a particular author or artist because the author/artist identifies so closely with the character."

I see your big "because" there, and raise you a "however": Sometimes an author just sucks at creating O Cs and uses the same one. I've used the same one for about 10 years now (granted, she never showed up in any of the stories I've posted because I never *finished* any of them), and of course she's evolved over that time, but it's just because I suck at OC's. They *always* turn into Mary Sues (the major reason I stop writing). I've taken two different Sue tests and they both say the current iteration isn't a Sue (yay!), but that won't mean she won't become one...

If anybody's interested in helping me, I'll take it. T_T And about the "constant use Sue" (Hmmm...), I'm just sayin'.