'Flawless' characters and Mary Sues are NOT synonyms any more than "Surprise Endings" and "Deus ex Machina" are synonyms.
(I would also point out that O-Chul had to be convinced to do things AND had to be confronted with the ramifications of his actions)
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I would also add that, strictly speaking, Mary Sues can ONLY exist in fan fiction as 'warping a pre-established universe to suit the authors wish fulfillment' is rather central to the whole point of Mary Sues.
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Edited to add:
The knock against 'flawless characters' is that they are allegedly uninteresting to read about. Yet one of the most iconic characters in all of American Fiction is nearly the text book definition of flawless: Superman.
And, yes, plenty of people have claimed he is a uninteresting character. Yet it perhaps is unsurprising that the author of OOTS is NOT one of the people who hold that opinion.
What makes Superman interesting is when he is put in a situation where his views are challenged and he had to come up with a solution that doesn't just involve smashing his way out.
In the same way a Whodunnit is about figuring out what the mystery is, what makes a Superman story good is seeing how he navigates the personal challenges while upholding his ideals and role model for others. In a sense, the story is not: Will Superman 'Win', but How Will Superman Solve the Problem in a Just Way?
Much like O-Chul, one might say.
NB: Character Development is not the same as Character Growth/Change.
Finding out what makes a character tick is character development as one literally finding out more about them. That is, the character is being developed/
Also, following the adventures in a character in a seminal point of their lives is also character development, as you see what a character did and why during important moments.
Did O-Chul 'change' at all during HtPGHS? Debatable. I would say that already present characteristics were refined. Which is one of the things that makes it a great story.