Aku

Aku (voiced by Makoto Iwamatsu) is the main antagonist of Samurai Jack, and is known as the shapeshifting master of darkness. He attacked young Jack's village, and built an empire with his rise to power. But Jack grew up to be a great samurai warrior and faced Aku with a magic sword. Before Jack could finish off Aku, he sent Jack into the future, where Jack was trying to find his way back to the past.

Trivia

 * Aku will become Jeffrey and Jaden's enemy in Jeffrey, Jaden & Friends meet Samurai Jack.
 * Aku is good friends with Set and Discord (What else can you expect from spirits of chaos?) and, to a certain degree, Pitch.