Aku

Aku is the main antagonist of the Samurai Jack animated series. He is a ruthless, diabolical, and manipulative demon who is the arch-nemesis of Samurai Jack. He is easily one of the most successful cartoon villains, as he managed to rule the world, and was unstoppable for years, as it took Samurai Jack 50+ years to defeat him. He is also the fragment of a black virus that is the true ultimate evil.

Aku tends to switch back and forth between being played for laughs, and being a truly threatening villain. He would either do something pettily "villainous" not to be taken too seriously, like telling stories to children to make them like him instead of Jack, or be extremely terrifying and dangerous, like causing genocide and enslaving innocents, depending on the episode.

In the first four seasons of Samurai Jack, he was voiced by the late Japanese-American actor, Mako Iwamatsu, who also voiced Happy Cat. In the fifth and final season, he was voiced by Greg Baldwin, who also voiced Frank Fontaine.