This may contain semi-spoilers from both games, or no spoilers at all, depending on how anal you are compared to me. Proceed ar your own risk.
Ok, so one of the main points of the story is that there's this kid, Sora, who can weild this awesome powerful weapon that has the ability to seal the worlds and all sorts of other stuff. They go on and on about how Sora was "chosen" by the keyblade, and how he's "the one" and how lucky and amazing he must be for the keyblade to have chosen him...
So why the hell do King Mickey, Riku/Ansem, and Kairi have keyblades too? Maybe I just completely missed the part where they told me.