Xehanort's Heartless is the main antagonist of Kingdom Hearts and Reverse Rebirth. In both these games he uses the name of his somebody's master: Ansem. He is, as his name implies, the Heartless of the series main villain Xehanort.
Ansem is first seen on Destiny Islands taunting Sora during the tutorials. Other than that he is not physically seen by Sora, Donald or Goofy until the final battle. After many intence showdowns with him, 'Ansem' opens the door to darkness, in attempt to use the power of Kingdom Hearts to gain the power of darkness. Sora then yells out: "Kingdom Hearts is LIGHT!" Light then purs of Ansem, destroying him.
However, because Riku, Sora's longtime friend served as the host for Ansem, a piece remains even after his destruction by the light of Kingdom Hearts. He is, however, defeated here as well, only this time by Riku.
Xehanort's Heartless is one of only three heartless with human-level intelligence, the other two being Sora's Heartless and Scar. He is a dark, cruel, emotionless sadist who enjoys watching his foes suffer.