Aku (Evilus sapiens) 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 septuplet sisters. Throughout seasons 1-4, Aku was voiced by Mako, but due to his death in 2006, Aku was voiced by w:c:samuraijack:Greg Baldwin in Season 5.
Aku constantly antagonized Samurai Jack: he often attacked him when he was weak, and other times defended himself from Jack's own gambits. Prior to Ashi sending Jack back in time to vanquish Aku's past self, the two seem doomed never to defeat each other, and while Jack had beaten Aku on numerous occasions, Aku merely transformed into a creature, retreated, and usually called out a taunt over his shoulder. Aku was entirely aware of his inability to defeat Jack and even lamp-shaded that in "Episode XLVIII: Jack versus Aku", which eventually discouraged him from fighting Jack further. Aku could have been considered the Deuteragonist of the series as a whole (albeit a rare, villainous one), as while Ashi became that in Season 5. Aku had the most focus in the narrative besides Jack.
While he's usually presented as a serious and threatening foe, Aku was also a source of comedy in the show, due to his outrageous design and sometimes wise-guy behavior, that was 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".
In Samurai Jack: Battle Through Time, Aku made a last ditch attempt effort to save his existence and secure his rule in the future.