What I think is: That you see in the end of KH2 that Mickey sends the group a letter of Xehanort's memory returning, so that might imply that they all visit him, and he recalls the whole story about Kingdom Hearts and the past. So you play from his perspective of view, and his history, also it might mean that you will control him as an protagonist. You may actually be a bad guy for once...But I don't know, it's just a theory.