For a large part of Kingdom Hearts, you have no clue who Ansem even is. In the middle of the game, you're told by Leon that Ansem was a great man that all the people of Leon's world loved. You're also told that Ansem tried mightily to stop the Heartless's invasion of Leon's world, but ultimately failed. Stopping Ansem doesn't mean that the Heartless have been defeated however, which opens the door for the upcoming Kingdom Hearts 2. "Those who know nothing understand nothing." - Ansem