Originating long ago in the depths of space as a formless evil Aku was banished by the gods Odin, Rah 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 fine well the warrior has the power to slay him, for this reason 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.
However, 50 years later, Jack, who had stopped aging due to a side-effect of time travel, finally achieved his mission and went back to his time period to destroy Aku, at the cost of his new love interest, Aku's half-human daughter Ashi.
PTE Redux Conditions: Aku is set to appear in Punch Time Exploders. Also, the new character Jacki won't exist in PTE as the Time Lords found a way for Ashi to exist after Aku's defeat.
Role in the series
Aku has not yet appeared in the spongebob or spyro series, yet he is considered due to the fact that Scroopfan used to have a picture of him in his old movie logo screen as supervideomaniac. currently, footage for him is still on a search. Thousands of years ago, Aku was one of Chernabog's 13 Seekers of Darkness. During the clash of blades, Aku's keyblade, Flames of Torment, clashed with that of a light keyblade, causing the x-blade to be forged. Now the Flames of Torment are currently hidden with it's merged light Keyblade in San Fransokyo. It's even revealed that after Aku's death, Jack found his time period's maternal ancestor of Ashi, the daughter of Aku he fell in love with and tried to marry before his life goal meant she couldn't exist anymore, whose original daughter betrayed her family to Aku because of his silver tongue, resulting in Ashi's perfectionist-trained maternal upbringing. Their modern descendant, Jacki, is now a samurai time lord.