In the Kingdom Hearts series who is the heartless who works with diz

There is no Heartless that works for DiZ. In Kingdom Hearts 2, the guy in the black cloak in the beginning with DiZ is Riku (which you'll find out how he got that way in KH re:chain of memories). His apprentice, Xehanort (who took over Terra's body in KH Birth By Sleep because he was old and wanted to live long enough to see his dark empire grow) (don't ask...you'll find out in BBS), took DiZ's research too far and created his own Heartless, stealing DiZ's old name (Ansem the Wise) and banishing DiZ to oblivion. Ansem is the Heartless's name, not the full title.