And since someone compared him to Him

Aku (voiced by Mako) is Samurai Jack’s main enemy. His name means “evil” in Japanese. He is similar to Akuma, the evil demon with burning eyes from Japanese mythology (which may also be another source for his name). He is an extremely powerful demonic wizard whose primary ability is shapeshifting like the god of evil Amatsu-Mikaboshi, though he possesses many other powers. He requires no food, water, or air and is capable of interstellar travel. He also has the ability to spy on Jack and others from a large sphere he can summon at will in his tower.

A significant aspect of the series is that Aku is immortal, and Jack’s samurai sword is the only weapon capable of harming and finally defeating him; even the slightest physical contact with the sword’s blade causes Aku severe pain, and wounds inflicted by it take much longer to heal from. Because of this, Aku does not like to fight Jack himself (only doing so when Jack is incapacitated or without the sword), preferring to let his minions do it for him. Aku is also vulnerable to varying degrees to other forms of magical or divine attacks (such as the powers and artifacts of gods).

Aku constantly antagonizes Jack, often attacking him while he is weak, and other times defending himself from Jack’s own gambits. The two seem doomed never to defeat each other, for though Jack has bested Aku on numerous occasions, Aku merely transforms into a small creature and escapes, usually calling out a taunt over his shoulder as he flees, a fact that he himself is aware of and even makes a reference to in one episode.

The episode The Birth of Evil reveals Aku’s origin. Long ago in the vastness of space, a great formless evil appeared. Before the darkness could do harm to the universe, it was set upon by the kings of three religions: Odin, the one-eyed king of Asgard and the Norse Gods; Ra, the sun god and king of the Gods of Egypt; and Rama, the seventh avatar of Vishnu, the supreme being in Hindu mythology. So fierce was their attack on the shadow, that it was completely destroyed, save for a small fragment that was flung aside in the heat of battle.

For ages the fragment drifted through the cosmos and eventually fell to Earth, and caused the impact event that wiped out the dinosaurs when it landed. The land around its impact site eventually formed into the islands of Japan, where it slowly grew and spread like poison over the course of eons, creating an ever-spreading forest of black spikes and toxic tar that devoured any who entered.

Eventually the forest grew so large that the Daimyo of the land (Jack’s father), decided to kill the evil at its source. Armed with a magic oil given to him by Buddhist monks, the Lord and his cavalry rode into the heart of the forest, the Lord himself the only one to survive. Once at the black lake at the forest’s center, the Samurai Lord doused an arrow into the oil he was given, lit it with a green flame, and shot it into the lake. Instead of destroying the evil however, the magic arrow gave it both a will and consciousness, thus the demon Aku was born. Aku proved to be unstoppable, so with the help of the three gods, Jack’s father forged a sword capable of harming him. With it, he was able to defeat Aku and somehow turn the demon into a black tree. This imprisonment lasted less than a decade however, as a solar eclipse released Aku upon the world once more.

While he is usually presented as a serious and threatening foe (as well as being pure evil), Aku is also a source of comedy due to his outrageous design and sometimes wise-guy behavior, supported by Mako’s over-the-top voice acting.