Aku

"Who dares to summon the Master of Masters, the Deliverer of Darkness, the Shogun of Sorrow, AKUUUUU!!!.""

- Aku before his first fight against Jack

Aku is the main antagonist of Samurai Jack, and the archenemy of the titular protagonist. He's the most dangerous villain in Samurai Jack,

Biography
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 not Jack, but his sword, knowing very well that the warrior has the sword of 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.

Personality
Aku is a purely dark soul. He is dominant, threatening, cunning and kills everyone who stand in his way or disobey him, no one have been able to resist his power, until the magic sword came as the only thing being able to hurt him.

Despite being the manifestion of pure evil, Aku is shown to have a sense of remorse. As such, he doesn't hurt children (though he killed a teenager once), instead he reads stories to them to make them root for him as the good guy, and writes Jack as a bad and frankly unashamed antagonist of his story.

Trivia

 * Aku is both comedic and threatening. Reading fairy tails for children yet still slaying 3000's of people on a regular basis.
 * He was voiced by Mako.