Aku

"What's this? It appears the Preschool Girls and I are being secretly monitored! But by who? Oh, no! Not... I--I--I can't say it! This is a villain so evil, so sinister, so horribly vile that even the utterance of his name strikes fear into the hearts of men! It's Aku."

--Su Ji-Hoon, The Peace Treaty

Aku is the main antagonist of the series, Samurai Jack. Born from a cosmic black mass of darkness, Aku is an evil, shapeshifting demon that has taken over the world and ruled it for countless eons. Throughout the series, he was voiced by Mako Iwamatsu in Samurai Jack seasons 1-4, but due to his death in 2006, he was replaced by Greg Baldwin in Season 5 of Samurai Jack.

Though he was capable of shifting his form into virtually anything he wished due to his metamorphic body, his standard form after self-upgrading and adapting to the Earth was roughly a humanoid being with a black body, a green mouth, and a red beard extending from the bottom of his chin. He had six horns, four long and two short, extending from his head, and eyebrows made of red, flickering flame. In his standard form, he also tends not to have legs, with his body simply ending at the bottom with robe-like darkness that sometimes manifested as flames. He was several stories tall. A deep droning noise was often heard around him when he appears.

Aku constantly antagonized Jack, often attacking him when he was weak, and other times defending himself from Jack's own gambits. Prior to Ashi sending Jack back in time to vanquish his past self, the two seem doomed never to defeat each other, and while Jack has beaten him on numerous occasions, Aku merely transformed into a creature and retreats, usually calling out a taunt over his shoulder, a fact that he was entirely aware of and even lampshaded in a Samurai Jack episode, "Episode XLVIII: Jack versus Aku," which eventually discouraged him from fighting Jack further. Aku could be considered the deuteragonist of the series as a whole (albeit a rare, villainous one), as while Ashi becomes this in Samurai Jack season 5, Aku had the most focus in the narrative besides Jack.