All through the game, I tried to figure out who the 7 princesses were, and my list did not match the reality. Alice was just a little girl, no royalty in her blood, so why is she a princess of heart? Because Wonderland made a good world to visit. And why isn't Ariel considered a princess of heart? She's actually a princess, and she's pretty darn tough, too.
But to get back on-topic, the whole theme of Kingdom Hearts (all 3 games) is strength of heart. That every heart is weak, every heart contains darkness, but that what separates the likes of Sora and the princesses from the likes of Maleficent and the Heartless is the inability to be overcome by those weak or dark points. Sora became a Heartless but did not lose his heart. According to KH lore, that *should* be impossible. Even by the end of KH2, there's no *concrete* answer as to why Sora retained his heart as a Heartless and why the other characters made it out of the battle the way that they did. You pretty much have to come to that conclusion on your own.
There's a good deal of mystery in this series, but it's not just for mystery's sake. The point is make you think about your self, to question what it means to *you* to have strength or darkness within your heart. The princesses weren't spiritually spotless (they fought in battle, too, even if not to the same degree as those fighting for them). Most good people in this world don't have spotless spirits, either. Ultimately, *that* is the lesson of Kingdom Hearts: choice. Darkness or lightness of the heart is ultimately determined by the choices we make.
As for what *is* this "darkness of the heart," I think that's a question that mankind has been asking for as long as it has existed. Personally, I don't think 'bad' thoughts automatically qualify a person as having 'darkness' in his/her heart. Like I said, it's about the choices you make. And like Sora said, "My heart may be weak, but it's not alone." This isn't an idea unique to this game or story, it's an idea that has been crucial to humanity for thousands of years.
Then again, maybe the princesses are just virgins. Or is that not what ya'll mean by "purity?"
