The role Riku was given was to be mysterious somewhat. He was given a role where he was compelled by the dakrness and turned evil, that's just his role, theres nothing a weak heart can do about it. Sora had a strong heart and didn't fall like Riku.
Plus Riku is just so awesome. And dude you must be thinking Riku from FFX and FFX-2 because This Riku in the game is awesome.
