Does Ginny count? By the sixth book, she started to get Poochie vibes, but I never really soured on her.
I know according to some Hermione fans she counts, but I liked her regardless.
I felt like she sort of popped up out of nowhere after doing zero for 3 books. Don't know if I can call her a mary sue if she really didn't do all that much before the 6th book.
Dub H
Crow T. Robot Captain Pixel: the Game Master
Does Batman count? I mean, preparation time can be handy and everything but it can only get you so far, specially when you fight supernatural entities like Batman does.
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Batman doesn't count, he's the star of the title, not a secondary character introduced to represent the author and be overpowered.
In Justice League stories though where he should be far outclassed by people on a tier above him, he often times comes out making near gods look like fools.