Final Fantasy

Kingdom Hearts: Chain of Memories / KHCoM

Final Fantasy XII Ashe
Ansem / Xehanort / Darkness in Zero
DiZThere has been a lot of confusion about who Ansem really is? Well Ansem is a Sage who lived in the Radiant Garden along with Kairi.
But if Ansem lived in the Radiant Garden then who is the Ansem in Kingdom Hearts? Well you see, Xehanort, one of Ansem's six deciples had lost his memories. He let Ansem perform an experiment on him thinking he might regain them, instead of getting them back he obtained the power of darkness. He performed experiments on people removing their hearts and such and created the Heartless and the Nobodies (The nobodies he created lead the Organisation XIII).
So Xehanort took on Ansem's name in Kingdom Hearts and he was the one who wrote the Asnem Reports, so Xehanort is the antagonist in Kingdom Hearts.
That's all good and well but the who the heck is DiZ? DiZ which stands for Darkness in Zero is the original Ansem, he discovered the Dark Corridor and infriltrated Castle Oblivion to discover what exactly Xehanort and his previous desiples where up to. As he learnt about the Nobodies and the Heartless he wrote the Ansem Reports which you get in Kingdom Hearts II.
So who did King Mickey meet, Xehanort or DiZ? He met DiZ.
I hope this helps clear up all your questions about who Ansem really is.