The episode The Birth of Evil reveals Aku's origins. Long ago in the vastness of space, a great, formless evil arose. Before it could do harm to the universe, it was set upon by the three prime deities of the Aesir, Egyptian, and Hindu mythologies: Odin the All Father, Ra the Sun God, and Vishnu, the Supreme God and Soul. During the battle, the deities' assault was fierce, and despite the entity's attempts to resist, it was almost entirely destroyed, save for a single piece flung from the whole. This fragment drifted through space for untold time, until it eventually crashed to Earth during the Cretaceous period of the Mesozoic era. It is implied that this crash is the event that caused the extinction of the none-avian dinosaurs, although the crash occurred in modern day Japan as opposed to the Chicxulub impactor, which touched ground in modern day Mexico. The fragment developed over eons into an ominous forest of black spikes that devoured any who dared enter. Eventually the forest grew so large that the Emperor (Jack's father) decided to kill the evil at its source. Armed with a magic oil given to him by Buddhist monks, the Emperor and his cavalry rode into the heart of the forest, the Emperor himself the only survivor as his cavalrymen vanished one by one. Once at the black lake in the forest's center, the Samurai Lord doused an arrow into the oil he was given, firing it into the black lake. Unexpectedly, the oil's magic did not destroy the evil, but had the unforeseen effect of galvanising the evil into a human-like form, giving it sentience and shape-shifting abilities, and thus giving birth to the demon wizard, Aku.
Aku easily defeated the Emperor, trapping him to the side of the tree from which Aku spawned in order to watch as Aku destroyed the Emperor's home. The Deities, sensing the good in the Samurai Lord, summoned him to a secret place. Taking the good inside him, they forged used avatar bodies to create an enchanted sword that is capable of harming Aku. Armed with the sword and a magical suit of armor, the Emperor plunged into battle against Aku. In the end, the Emperor was victorious, but he was unable to destroy the demon, and so instead sealed him into the devestated wasteland. As he was sealed away, Aku vowed he would one day return. Shortly after, the Emperor's wife gave birth to a son. Determined to learn from the catastrophe, and aware that Aku may indeed one day return, the Emperor and his family determined on creating a plan, and thus orchestrated the events that would set Jack on his eventual years of training.
Fifty years after Samurai Jack arrived in the future, Aku managed to destroy almost all of the world's time portals. With Jack's ability to travel back to his own time seemingly eliminated, Aku decided to stop confronting Jack and simply wait for the samurai to die of old age. However, it later became apparent that Jack's initial time-travelling had halted his natural aging process, leaving him forever at the age that he was when he first went through the time portal. This discovery scuttled Aku's plan and left him at somewhat of a loss as to what to do about his now timeless foe.
In Episode XCVI, while sitting on his throne, Aku was informed by a henchman that his lair was under attack by a rebel army led by the an aged Scotsman. Aku decided to deal with the rebels himself, as he saw it as a chance to escape from his depression. Aku then flew out of his lair and shape-shifted into a giant cannon ball, effortlessly slaughtering most of the army. Although his enemies were destroyed, Aku quickly fell back into depression, as the invasion had failed to provide even the smallest challenge. He was then confronted by the Scotsman, who insulted and ridiculed Aku, stating that Samurai Jack has inspired many to oppose him, only for Aku to shoot him with his eye beams, killing him instantly. Aku, with a bored tone, could only wonder why the Scotsman mentioned Jack before returning to his lair. However, unknown to Aku, the Scotsman returns as a ghost and instructs his army (who are all his daughters as well) to continue to amass their forces and find Samurai Jack.
In Episode XCVIII, in a flashback, it is shown that Aku pulled Jack out of a time portal just as he was about to reach the past and then obliterated it, taunting Jack by telling him that said time portal was the last in existence. In his rage, Jack tried to destroy Aku once and for all, although Aku backed out of the fight and transformed the Rams that had been helping Jack into mindless beasts that he soon killed, causing him to accidentally drop his sword into a pit in, much to his horror.
In Episode C, Scaramouche managed to reach Aku and report the status of Jack's sword. Excited with the news, a reinvigorated Aku confronted Jack for the first time in decades, only to discover the samurai had already recovered his weapon. After destroying Scaramouche in anger, Aku casually attempted to leave the area until he noticed a familiar smell coming from Jack's companion, Ashi. Suddenly remembering the time he visited the Cult of Aku and offered them some of his essence, Aku quickly deduced that Ashi was his biological daughter and used the essence inside her to control her into fighting Jack, casually remarking to a giant robot corpse how his (last remaining) daughter had fallen for his mortal enemy. Already bored with Jack's attempt to free her, Aku simply tossed the robot onto them before corrupting Ashi with a new bodysuit in his likeness. When Jack couldn't bring himself to kill Ashi and surrendered, Aku ordered his daughter to stand down, claiming Jack's sword for himself and relishing in his victory.
In Episode CI, Aku announces and broadcasted to the world that he is victorious as he is holding Samurai Jack hostage and stolen his sword. He warns them not to interfere as he is about to kill Jack in front of everyone, but he begins pondering on how to do it. After a period of in decision, Aku decides it's best for Ashi to kill Jack. As she was ready to lay the final blow to Jack much to Aku's delight; most of the characters that Jack has encountered in the future came to his aid and attack Aku. Aku begins to do a great battle with them while Jack fights the corrupted Ashi. Aku finds amusement in the armies attempts to save Jack, as while they may be able to fight off Aku's forces, Aku himself is nearly untouchable. He checks on his daughter Ashi to see if she has killed Samurai Jack. But thanks to Jack, Ashi has free herself from Aku's corruption and stands up against him. The furious Aku attacks her but is surprised to see that she has Aku's powers and thus she used it to summon a portal and send both herself and Jack back to the past. The future Aku begins to worry that his defeat is immediate as he is about to be erased from existance.
Back in the past after Jack's first battle against Aku; the demon has just send Samurai Jack to the future. After he finishes his speech of meeting him again in the future, he was shocked to see that the future Jack and Ashi has returned on that same spot. As Aku tries to process why he is back so suddenly, Jack immediately charges and begins his short final showdown with Aku. Jack is finally successful in almost destroying Aku once and for all. As the injured Aku tries to retreat, Jack manages to stab his severed head with his katana and the demon merges into the sword before Jack slams his sword to the ground. At long last, Aku was finally destroyed once and for all when he and his lair begins to explode; avengeing all the deaths and suffering he has commited and ending his tyranny forever. However, Aku's death came at a price when his daughter Ashi was erased from existance right before Jack was about to marry her.
- Aku is voiced by the late Mako Iwamatsu (Seasons 1 to 4) and Greg Baldwin (Season 5).
- Aku is also Tirek's partner.
- Aku is also one of Bowser's partners due to that he's the Master of Darkness.
- Aku became the antagonist in The Night to Remember.
- Aku is one of the few Cartoon Network villains who has actually taken over the world. However, in the Pooh's Adventures trilogy he is not ruler of the world.
- Aku serves as the leader of the Council of Shadows in the Logan's Adventures series.
- At the end of the series, Mistress 9 will revive Aku with all of is powers (excluding the Time Portal power) and vows revenge on Pooh and Pals, Samurai Jack, and Ashi and joins The Masters of Evil.
- Aku is one of the most trusted allies of the Grand Monarch of Evil: Dark Specter.
- Aku is also Anubis' friend and one of the Masters of the Heylin side as well as one of Princess Twilight and The Rainbooms' most hated, dangerous and powerful foes all along in Princess Twilight Sparkle and The Rainbooms' Adventures.