Jack-Aku first appeared in Episode XLIII: The Aku Infection. He has Aku's black skin, horns, great flaming eyebrows, green lips, fangs but doesn't have the red beard Aku has.
The Aku persona initially caused subtle changes to Jack's personality before outright forcing him to commit evil acts against his will. Like the real Aku, Jack-Aku is cruel, violent, and insane and attempts to torment Jack by impeding his quest and hurting those around him.
Jack-Aku spawned from an infection a sickly Aku had unwittingly inflicted upon Jack during one of their many battles atop a snowy mountain. Aku, citing illness, left before Jack had a chance to actually engage him, leaving Jack alone to continue his journey. Over the next two days, Aku's essence slowly began to corrupt Jack without his knowledge, resulting in Jack stealing a bag of gems without realizing it, after which Jack bartered with a man's life in exchange for money. During all this, Jack's skin began to turn black, first forming on his arms before slowly making its way throughout his body as the illness started taking a greater hold on him. Eventually, Jack wandered into a town at the foot of the mountain, where he killed an innocent robot after the two of them bumped into each other. Realizing what he had done, Jack fled, and stumbled over to a water barrel, where he saw that half of his face now resembled Aku's own. Believing that the Lizard Monks could help him, he quickly made his way to their temple.
Eventually, Jack arrived at the temple to seek help from Master Ning. When he had arrived, the infection had consumed most of the samurai's original body, as well as his mind. Almost immediately, the Aku personality forced Jack to attack the Lizard Monks and destroy the time portal in the temple, much to Jack's horror and shame. Jack-Aku was then subdued by Master Ning as Jack begged him for forgiveness.
Master Ning had Jack restrained over the pool, while the rest of the monks prayed for Jack in order to hold the infection at bay. Ultimately however, it fell upon Jack to exorcise the evil from his body. Jack did not believe that he was capable of defeating the sickness alone, and he begged Master Ning to help him somehow, but Jack suddenly succumbed to the Aku persona completely. But within Jack's soul, the memories of his parents, all the people he had helped, and all the allies he had made, manifested themselves as a mighty fortress which Jack used to strengthen himself.
With the help of Jack's parents and with the blessings of all his friends and allies, Jack was able to banish the evil from his soul, and the infection was completely cured.
He presumably never came into existence since Jack has destroyed the past Aku.
Skills and Abilities
He appeared primarily to simply possess Jack's own martial prowess. However, while he was able to wield Jack's sword, he appeared not to be able to inflict lethal damage with it. Due to its nature, Jack's sword was not capable of being used for evil, and the Lizard Monks whom he struck with it appeared to have simply been knocked unconscious. This stipulation appeared only to affect biological organisms however, as, though he appeared not to be able to kill the Lizard Monks, he did kill (or at least very badly damage) an innocent robot at one point.
- Jack-Aku is similar to Mad Jack, being a dark counterpart of him created from Jack's own inner darkness that is only defeated when Jack regains his resolve and cleanses his own spirit. However, while Mad Jack is purely an embodiment of Jack's anger and frustration that requires Jack to come to terms with his own feelings, Jack-Aku was created from a piece of Aku's own essence infecting Jack and was destroyed by Jack focusing on all the good deeds he had done in the future.
- Jack-Aku is also similar to the Daughters of Aku, being a human-demon hybrid created when Aku's essence fused with a human. However, their condition is genetic, whereas Jack-Aku is an infection. In particular, Ashi's corruption in Episode C and subsequent struggle to free herself from Aku's influence is very similar to the trials Jack underwent to purge himself of Aku's corruption.