If you notice, riku always keep his left arm up against his side, and ALWAYS uses his right hand. My guess is that it was damaged during his fight with Roxas. I think when roxas came with the oathkeeper and Riku swung him away, he bent his arm in a way the human arm can't bend and broke. They didn't show him fighting after that scene, and it was probably cured during the time he was in his ansem state. Thats my theory, and didn't Nomura say the nights didn't have a connection to kh1 and kh2? Wasn't it a new project or something.
All these reports and ultimania are pretty random, but cool, so thanks.
