(I am both young and old Xehanort ) 
Xehanort: I am the master of this whole plan I took terras body to watch the key blade war be fought and see the out come I vowed to see it so I chose one boy to make the X-blade so I tried the boy but sadly his kindness was much so I took the dark half of and it made a boy so I called him @_vanitas and took him under my wing and trained him and he stays by my side the whole time till ventus got rid of him by destroying him

young Xehanort: I make sure that for my old self everything goes as planed so I gave up my body and sent my heart back in time and sora being the slow idiot he is never noticed the X on his chest to keep track of him and he become are next member and be the 13 one but that @_axel_ guy changed sides and saved him so I will get him and he will be are13 member