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The way I see it, the keyblade goes to whoever has the strongest heart. It began with Mickey...for obvious reasons...but his version was inverted from the Kingdom Key we've come to know. Riku was next in line to recieve it, but as you said, he turned to darkness. So the keyblade then fell into Sora's hands (obviously, Roxas can wield it because of who he is, so we won't discuss that). But as far as I remember, the only person who reffered to the title as "Keyblade Master" was Riku in the first game, when he said, "Only the keyblade master can open the secret door, and change the world." Everyone else used the term Keybearer.
And I agree that Kairi can't really be considered a Keybearer. She had it for, what, ten seconds?

Once again, that term...Keyblade Master...it's inappropriate. Like I said, only Riku called it that. And that was because he felt that he was more worthy than Sora because it was meant for him, and because he felt his heart was stronger.
And technically, Riku was already in darkness before that. At least the way I see it. He was willing to do anything he could to get off that island. As Ansem said, "He sought a way to cross over into other worlds, and he opened his heart to darkness." That doesn't have to mean he needs to be completely submerged in darkness, but he opened his heart to it. And once that happened, the keyblade deemed Riku unworthy, and turned to Sora. And as we all know, they were working on that raft before the time when the game starts. So Riku must have opened his heart to darkness before Sora ever entered that strange place.
As for Kairi, I disagree. Like I said, the keyblade goes to those with the strongest and purest hearts. So of course it wouldn't deny Kairi; she's one of the princesses of heart. Does that mean the other six princesses could hold a keyblade? I don't know. And I doubt I, or anyone else, ever will. I still don't consider her a Keybearer because she only used it for ten seconds. I consider Riku a keybearer because that's who he was supposed to be, and ended up being. And I consider Sora a keybearer because he was the one who went through two game with it. The same applies to Mickey. But a ten-second usage...I'm sorry, no. If a burglar were to break into your house, you picking up a katana and killing him would not make you a samurai.

Okay, you did it. My KH Fanboy blood is boiling. Prepare yourself, mortal.
*deep breath*

To begin, of course it chose Riku first. Wikipedia, the game packet inside the game case, every source imaginable have confirmed that Riku is the original keybearer, so your first two points are of no significance.
Secondly, he got that keyblade from the captive hearts of all of the princesses of heart except Kairi's, which - correct me if I'm wrong - sounds fairly dark. And remember Ansem said that Sora's heart was stronger "for that instant", and that was why he regained the keyblade. But when Riku merged with Ansem and became stronger, the combination of stength and darkness of his heart created that keyblade. And since he was still the original keybearer, he couldn't be denied it. Second point = pointless.
And lastly, you shouldn't argue about a game that has only been hinted at by the creator, but if you must, then you might as well know that the only hints Tetsuya Nomura has made at KHIII imply that it will be along a different timeline, which means it's more than likely that Sora, Riku, and Kairi will play very little role in it if they're in it at all. But since it isn't OUT YET, your argument is null and void.

DeadJib: 3
The Legendary Destroyer: 0!
YOU LOSE! GOOD DAY SIR!

ok you 2 carm down you dont need to goon now both of you stop this pointless argument or i will ban you both

Ether way you only need to ancer the question that was asked by OneWingedAngel55 and not add on to it every one has there own point of few people do not need to tell other people what they belive is right