When Roxas tries to do what Xion has asked of him, Riku hears Xion’s voice asking him to stop Roxas. Why?
Nomura: First he have to look at things in order. First, the reason why Roxas throws a keyblade to Riku, and we then see a vision of Xion, is that there is a small remaining part of Xion in Roxas that wants to stop him, and makes him take those actions. The name of the keyblade that is given to Riku suggests that it has something to do with Xion. Xion wants Roxas to set Kingdom Hearts free, but doesn’t want him to face Xemnas right now. She sees that he would most likely lose. So she begs Riku to stop him.