Ansem The Wise wrote Ansem Report #0 during KH I. Xehanort wrote the rest, which in the american version, turned out to be 1-13, all bearing the name Ansem, the Seeker of Darkness by the 13 penned by Xehanort.
DiZ (Ansem The Wise) wrote the Ansem Reports during KHII: Ansem Reports 1-12. It all ended with a bang, and Ansem the Wise was gone, leaving many questions unanswered.