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The thing is he seems mindless, he's supposed to be pure darkness, but he doesn't seem all that crazy and an independent person. I mean characters with light in their hearts are more evil and crazier than him, he just follows orders, he's a lapdog to Xehanort.Is that really what you think? You're half right, there, he's clearly mentally unstable, but there's been many mentally unstable villains in history, The Joker, Green Goblin, Kefka, Sephiroth. I think he'll end up being the Kingdom Hearts equivalent of Kefka, no goal, but to destroy everything. And, I love crazy villains, their laughter, their speeches, motivation, and being crazy keeps them from having moral views. Seeing Vanitas just lose it with no master would be awesome.
From Wat they've shown us, I think they will both be passed as keyblade masters. However, it will be interesting to see them fight against each other.
personally I think Sora can beat riku. He has versatility to keep riku on his feet through the use of magic and dual-wielding. I think with these aspects alone and the fact that riku has lost his dark powers (which aided him in defeating roxas) Sora is ahead of riku by a small margin.
It's not known if they will fight or not. The test may be the same as the one Terra and Aqua received, but it also may not. And maybe Riku has some special power he hasn't showed yet (...doubt it )