Mechanics based Personality
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For a roleplaying game, D&D provides scant material for defining your character's personality, other than his "alignment". With other RPGs, you can pick from lists of virtues, flaws, quirks, mental disadvantages, background options, etc., to flesh out your character, making him truly 3-dimensional and roleplay-able. But D&D doesn't have any of that built into its core rulebooks.
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If you absolutely must have a D20 process for creating a personality for your character, WOTC has the Hero Builder's Guidebook.
If you find that insufficiently pompous for your taste, go pick an archetype from a Storyteller system book and describe your Personality In One Capitalized Word.
If you really want to be pretentious, every comprehensive book on fiction writing has a chapter or more on character creation and representation.
Myself, I can't possibly tell you how grateful I am Wizards of the Coast felt it was unnecessary to bloat their books with White Wolf style paroxysms of joy about how they've reinvented roleplaying by making it all gloomy and angstful and Capitalized so you need to play the game with this sort of character in this sort of way unless you want to miss out on the True Roleplaying Experience.
Please excuse the structure of the last sentence.
Seriously, if you know how to categorize human personality well enough to make a good list of quirks and provide systems for them, there's a Nobel Prize waiting for you. Stop wasting your time posting here and claim it.
If you know how to model the personalities of the human, demi-human, semi-human, and not human at all, you are probably the most valuable person on the planet, and should be preserved for a "first contact" situation.
Even Storyteller games suggest you concieve of the personality first, then choose Merits or Flaws to support aspects of the personality, not pick a neat Merit, then tweak your character to have it.
The conception of the character always drives the mechanics, not the other way around.