Aku is the main antagonist of the Samurai Jack series. He is a ruthless, diabolical, and manipulative demon who is the archenemy of Samurai Jack. He is easily one of the most successful cartoon villains, as he managed to rule the universe, and has been unstoppable for years.
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.