Aku

"Long ago in a distant land, I, Aku, the shape-shifting Master of Darkness, unleashed an unspeakable evil! But a foolish Samurai warrior wielding a magic sword stepped forth to oppose me. Before the final blow was struck, I tore open a portal in time and flung him into the future, where my evil is law! Now the fool seeks to return to the past, and undo the future that is Aku!"

- Aku

Aku is the main antagonist of Samurai Jack. He is a dominant, mysterious, brooding, dark-souled omnipotential demon who is an extremly competent archenemy to Jack. He is easily one of the most succesful cartoon villains, as he managed to rule the world, and has been unstoppable for years.

At first, Aku was often known for being pure evil. However, it is later implied that he is actually created by a black virus that is the true ultimate evil.

Origins
Originating long ago in the depths of space as a formless evil Aku was banished by the gods Odin, Ra, and Vishnu in order to protect the universe from his influence - yet a small shard of the great shadow managed to survive and flew off into space - for many years this shard continued flying through space until it eventually landed on Earth, causing the extinction of the dinosaurs in the process.

Many years later the site on which Aku landed would become the ancient island of Japan, which Aku infected like a horrible disease - creating vast forests of evil in the land and threatening to consume it, eventually Aku's threat became so great that Jack's father decided to rid the land of the demon once and for all using magical-oil made by Buddhist monks, however when he fired an arrow coated with this oil into the evil entity it didn't destroy it but rather gave it form and sentience, thus forming Aku as a humanoid threat of unstoppable power.

However with aid from Odin, Ra, and Vishnu Jack's father created a magical-sword powerful enough to defeat Aku and fought the demon, at the end of the conflict Aku was transformed into a tree - yet the imprisonment wouldn't last much more than a decade before a solar eclipse freed Aku again.

Aku would then proceed to lay waste to the kingdom Jack's father ruled and engaged in a battle with Jack himself - Jack very nearly killed Aku for good using the magical-sword but Aku threw open a portal in time and transported Jack to a future-world in which Aku ruled supreme.

For the rest of the series Aku is portrayed as the all-powerful ruler of the world and is an almost unstoppable force - yet he fears Jack, knowing very well that the warrior has the power to slay him, Aku has placed a high bounty on Jack's head and is forever trying to dispose of him, whether it be with manipulation, force, or trickery.

Powers and Abilities
Shapeshifting: Called the "Shape Shifting Master of Darkness" at the opening of each episode, Aku can change into virtually any form he desires, from a giant dragon to a small insect, or even a beautiful woman. He can also change his size, liquefy himself, stretch, and expand his body. Aku's consciousness can exist even in a part of himself. He typically changes into animal forms that are threatening to fit certain situations, such as a scorpion or an octopus. He is however limited to his own color scheme, no matter what form he takes it will always be black, red, and green.

Aku needs no food, drink, or air and is capable of interstellar travel. He also has the ability to spy on Jack through a large sphere he can summon at will in his tower.

As Aku is a great and powerful wizard, he possesses a wide range and knowledge of magic that varies from episode to episode as the series demands. His powers, as shown thus far in the series include: A significant aspect in the series is that Aku is ageless and immortal. Aku's body is immune to nearly all forms of physical injury. If wounded, the wounds inflicted upon him will regenerate in moments. He can only be harmed and finally defeated by Jack's samurai sword. It is notable that Aku is also vulnerable to various magical and divine attacks (such as the powers and artifacts of gods).
 * Superhuman Strength: When Aku challenged Jack to a so called final match he was shown to have superhuman strength even in a somewhat human form.
 * Limited Invulnerability: Aku has shown the ability easily withstand weapons of any sort, such as simply absorbing arrows shot at him or not even being fazed after being hit by torpedoes as well as being capable of breathing underwater and existing in the depths of space, however he was shown to be vulnerable against mystic weapons and the weapons and artifacts of Deities and especially Jack's blade which was forged through the very essence of purity within Jack's father by the Deities Odin, Ra, and Vishnu.
 * Fire Generation: Along with being capable of breathing fire Aku has also been shown creating it such as in one comical situation he had to re-ignite his eyebrows.
 * Time Travel: This was the very ability that Aku had used to send Jack into the future at the beginning of the series, though so far it has only been used once and was a one way portal forward in time.
 * Laser Eye Beams: Aku used this ability in numerous battles with Jack and he also used it to destroy the majority of Jack's Kingdom.
 * Teleportation: Aku uses this quite often to appear before those he has business with, such as the Imakandi.
 * Summoning Storms: Aku is capable of conjuring storms through will.
 * Magic Materialization: The ability to create objects out of thin air. He once used this ability to conjure gold, jewels, and treasure for the Imakandi.
 * Psychokinesis: Aku has shown the ability to move objects with his mind alone.
 * Sonic Scream: Aku was shown capable of producing a powerful scream that causes massive damage to the landscape.
 * Necromancy: Aku has been shown in one episode to be capable of animating countless dead bodies, and bending them to his will as they have neither mind nor soul.
 * Premonitions: On one occasion, Aku had a dream that Jack found a way to return to the past, and realized it was no dream but a premonition, which spurred him into successfully preventing it from happening.

Although Aku can transform into any form, his reflection will show his normal form, most noticeable when Aku turns into a hermit, and asks Jack to look for the god's artifacts. When Jack and Aku are in the swamp, Aku shows his normal form even when he is transformed into his hermit form. He will always have the same reflection, so he splashes the water, and uses the ripples to hide his reflection.

Weaknesses
Despite his nearly omnipotent abilities, Aku has a few weaknesses. These include:
 * His shape shifting is limited to his red, green, and black color scheme, also, his reflection will show his normal form, no matter what guise he takes.
 * He can be harmed by magical objects and attacks like Jack's father's magical-sword.
 * In one episode (XLIII), he appears to have somehow caught the flu.

Personality
Aku is evil incarnate, he embodies the very essence of an evil overlord. Cold and dominant to all he meets, it's obey or be obliterated with Aku. Arrogant and sadistic by default, he has been the ruler of most of the planet for hundreds of years and does not take humiliations easily. Although without any mercy he has enough of an understanding of humanity to successfully manipulate and control people.

Despite this he possess a dry and witty sense of humor and often jokes when in a good mood, frequently mocking Jack whenever he can with whatever he can. In Aku's Fairy Tales it's shown that he seems possibly unwilling to cause harm to children and tried to make himself look like a hero to them rather than Jack. He failed to do so and left the children alone. He did, however, vaporize an alien kings' young son for calling him a 'monster'. Aku sent him to the Pit of Hate (an implied torture chamber).

Trivia

 * He was voiced by both the late legendary Mako Iwamatsu in the Samurai Jack series and Greg Baldwin in Cartoon Network Universe: FusionFall.