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Summary
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 earth and ruled for countless eons. Throughout the series, Aku was voiced by Mako Iwamatsu in seasons 1-4, but due to Mako's death in 2006.

Though Aku was capable of shifting his form into virtually anything that he wished due to his metamorphic body, his standard form after he self-upgraded and adapted to the Earth was roughly a humanoid being with a black body, a green mouth, and a red beard which extended from the bottom of his chin. Aku had six horns, four long and two short, that extended from his head, and his eyebrows were made of red, flickering flame. Aku typically assumes a humanoid form, concealing his entire lower body in a massive, thick robe that sometimes manifested as flames. There is one known instance where Aku removes the robe to reveal his thin legs in "Jack and the Zombies". Aku can stand several stories tall and a deep droning noise was often heard around him when he appeared.

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".