Aku is the central antagonist of the cartoon Samurai Jack. He is also known as the "Master of Masters", "The Deliverer of Darkness", and the "Shogun of Sorrow". He was voiced by the late Mako Iwamatsu, who also voiced Master Offay from Super Robot Monkey Team Hyperforce Go! and General Iroh from the first and second seasons of Avatar: The Last Airbender.
Aku constantly antagonizes Jack, often attacking him when he is weak, and other times defending himself from Jack's own gambits. The two seem doomed never to defeat each other, for though Jack has defeated him on numerous occasions, Aku merely transforms into a creature and escapes, usually calling out a taunt over his shoulder, a fact that he is entirely aware of and even makes a reference to in "Episode XLVIII: Jack versus Aku".
While he's usually presented as a serious and threatening foe, Aku is also a source of comedy in the show, due to his outrageous design and sometimes wise-guy behavior, supported by Mako's over the top voice-acting.
Aku means "Evil" in Japanese. It is written as "悪" in Kanji. The kanji can also be pronounced as "Waru".