(SPOILER)You are correct. DiZ is Ansem the Wise, the ruler of Hallow Bastion, who was studying the Heartless. The Ansem that you fight in the original Kingdom Hearts is actually Xehanort's Heartless. Xemnas, the Superior of Organization XIII, is the Nobody of Xehanort. Xehanort was the foremost apprentice of Ansem the Wise, until he turned against him, stole his research, and lost his heart. Roxas was the Nobody created when Sora became a Heartless in Hallow Bastion. And Namine is Kairi's Nobody, and was created when Kairi's heart was locked in Sora's.