Aku

Aku is the shape-shifting master of evil, born long ago from one small splinter of an evil darkness that once threatened the universe. After receiving sentience, Aku quickly lashed out and spread his evil to take over the world. The only thing that could hold him back was a magic sword forged by the gods to defeat him. When at last the fated battle arrived and Aku neared defeat, he sent his foe into the future where Aku's evil was law. Now Aku seeks to end the samurai and undo the possibility of defeat.
=History=
Aku- Samurai Jack Wiki
=Personality=
Aku is the absolute personification of evil, formed from pure black darkness that was sparked with sentience. From the beginning, Aku was a clever and ambitious schemer who had his sights set on world domination. He very quickly was able to conquer his surroundings and move on to the whole world, enslaving those all around him. But in spite of his absolute evil, Aku never killed when he could avoid it. Even his worst enemies, the parents of Samurai Jack, were allowed to live as slaves working towards building his empire. This is because Aku's desire is not to kill or destroy, but to simply control everything and to be feared and respected. This can be evidenced by the fact that when Samurai Jack became more popular among his subjects than himself, Aku tried to peacefully manipulate the minds of children to believe Aku was the real hero and that Jack was the oppressor. 
His preferred manner of ruling is done through great shows of power by taking on large impressive forms in order to inspire awe and fear. However when these methods fail, Aku has demonstrated a craftier side and has used many schemes, tricks, and lies in attempt to get the better of his more dangerous foes like Jack. He has little difficulty in adopting an entirely new persona and keeping up the act for as long as needed unti lhe achieves his goals. And when he achieves his goals, he loves to gloat over the falures of his foes and mock their mistakes. As a result, he often comes off being very over the top when dealing with others, whether they are his subjects or his enemies.
==Strengths==
Aku is a powerful shape shifter composed of a vast amount of black, inky concentrated evil. As a result, he can shapeshift into a wide, wide variety of shapes that is limited only by own imagination. Typically these creatures resemble him in form (typically maintaining his fiery eyebrows and beard), but if he tries hard enough, he can completely alter his form to look nothing like himself, demonstrated when he was able to take the form of a woman and another time as an old swamp hermit. He spends most of his time in his large human-shaped form where he prefers to appear massive (about fifty feet in height) in order to make himself appear intimidating. He can also separate himself into separate entities, at one time literally becoming an army of human-shaped Aku-soldiers in order to do battle. He is utterly immune to most weapons and absorbs attacks without so much as flinching. Although he doesn't often need to, he also possesses numerous magical abilities such as fire generation, laser eye beams, teleportation, psychokinesis, and necromancy. 
In addition to his impressive array of powers, Aku has a certain amount of cunning. He has ruled the world for thousands of years now without ever been toppled in all that time. Rather than allow the world to stagnate, he has been shown to take a keen interest in technological progress, demonstrated by the fact he uses vast robot armies to keep the world under his control. Consequently he is familiar with advanced technology, even if he doesn't really know how they work. Perhaps his greatest strength is that he knows when to retreat, as he has often done in battles with Samurai Jack. If his plots or schemes have failed, he won't bother to stick around to defeat Jack himself, as he is well aware of his own limits. 
==Weaknesses==
Aku's primary weakness is the sword carried by Samurai Jack, which is said to be the only thing that can kill him. However he is not completely invulnerable and has been shown to avoid other god or god-like beings who have the power to at least repel him. Holy magic or weapons enchanted by some sort of holy magic can harm him as well, even if it's not enough to kill him completely. Despite his impressive shapeshifting talents, one weakness is that even when taking a form that's nothing like his normal self, he will appear in reflections as Aku. Small animals also tend to be aware of his evil and immediately flee from his presence. As a combatant, Aku is not the most proficient of fighters and usually relies on overwhelming force to defeat his enemies. Despite his many talents, he has never been victorious in a fight with Samurai Jack, usually being forced into a retreat when the battle goes poorly.
Despite Aku's cunning, he doesn't always think through his plans. Though sending Jack into the far future might have been a good move, it did only delay an inevitable battle between the two. He also has a tendency to get caught up in gloating or mocking his foe, that often he gives his enemies time to escape or get the better of him before he notices his mistake. Because he is so used to ruling everything, he's very out of touch with new people, evidenced by his attempts to tell fairy tales to children and ffailing miserably in doing so.