Aku

Aku is a villain from the Cartoon Network universe. He may be one of the most powerful villains in Cartoon Network because he cannot be hurt by anything expect for weapons of a divine caliber, like his archenemy Samurai Jack's sword.

Aku is voiced by Mako Iwamatsu.

Legends of the Multi-Universe (Menslady's story)
He first served Jack of Blades but now serves Galvatron.