Aku is the main antagonist of the series Samurai Jack. Born from The Black Mass of Darkness, Aku was an evil, shape-shifting demon who took over the world and ruled it for countless eons, the arch-nemesis of Samurai Jack and the biological father of Ashi and her twin sisters. Throughout the series, Aku was voiced by Mako Iwamatsu in seasons 1-4, but due to Mako's death in 2006, Aku was voiced by Greg Baldwin in Season 5.