Aku (voiced by the late Makoto Iwamatsu, currently by Greg Baldwin) 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. For 50 years Aku ruled with an iron fist, and made various attempts to stop Samurai Jack, only to end in failure. Aku eventually created Ashi, but eventually, he finally met his demise, when Ashi and Jack returned to the moment when Aku sent him to the future with Ashi, and was finally defeated by him, never to be seen again.
- 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.
- Aku will become Twilight's enemy in Twilight meets Samurai Jack, and he is a member of Anubis' Heylin Empire to steal the Rainbooms' magic and annihilate them in Twilight's Adventures series.
- Aku will become an enemy of the Miztyk Knights in the future. In the series, Aku is revived by Lucifer, and made a member of the Kingdom of Darkness.