Mad Jack is an antagonist in Samurai Jack. He is the embodiment of Jack's inner darkness, temporarily given form by Aku to defeat him using his own skills against him. He made his first appearance in Episode VIII: Jack versus Mad Jack. He lost his physical form after Jack found peace within himself and caused him to disperse. When Aku destroyed the last remaining time portal, Jack's inner anger and frustration caused Mad Jack to resurface as Inner Jack, where he continued to antagonize Jack in his mind during most of Season 5.
Aku had sent several bounty hunters to attack Jack, all of which failed, Aku lamented that no fighting style seemed to be able to beat Jack's, which caused him to form a plan; to pit him against himself.
After having to fight several bounty hunters, Jack became infuriated at his lack of peace and he was tired of fighting. He tried to calm down after defeating the last of them, but when his sandal strap broke, he snapped and threw the sandal at one of his wanted screens. The screen then started to light up, energy came from it and formed around the sandal, sealing the strap, until it finally forming into a more sinister Jack, Mad Jack. Surprised Jack questioned who the double was. In response, Mad Jack revealed himself to be the spawn of Aku's magic, formed from Jack's own hatred as well as being his dark side possessing all of his skills, created for the sole purpose to destroy Jack for Aku.
The two fighters then begin battle and appear to be evenly matched, trading blow for blow. However, since they both possessed equal skills, neither one could best the other. Their fight grew more intense as they continued, forcing each other's hair to sprawl out, reducing trees to ashes with the sparks of their swords, and outright destroying the area with the force of their blows.
As Jack struggled with his dark half, he saw the reflection of his own eyes in his sword and to his horror saw they bore great resemblance. Finding his peaceful center, Jack stepped back and calmed himself, causing all of his negative emotions to fade away. He then claimed the fight was over, stating that the negative emotions that formed Mad Jack were gone, meaning Mad Jack didn't exist.
Refusing to let Jack look down on him, Mad Jack boasted that he was real and charged toward Jack to deliver the final blow. However, while Jack remained in a meditative position, not even bothering to defend. As he was to strike Jack down, Mad Jack was instantly in Jack's control, able to do nothing except scream in impotent fury and defeat before he was sealed away by Jack.
At some point, Mad Jack was the cause of Samurai Jack losing his katana. As Aku destroyed the final time portal, Jack's frustration began to surface, causing his rage to take over as Mad Jack and angrily lash out on Aku. Mad Jack then causes Jack to kill the goats; weakening Jack but allowing him to come back to his senses. As soon as a large slab hit Jack, the katana fell into the pit and was never seen again. Mad Jack would continue to torment Jack mentally by resurfacing the other negative personalities to continuously provoke him into giving up on his quest, citing the loss of their sword as proof of their failure and dishonor to their ancestors.
When Jack tried to make a perfect tea but failed, Mad Jack manifests himself once again frustrated at the monk for not telling the location of the Katana. But Mad Jack's attempts at interrogating the Monk are met with Jack confronting him about being the reason why he lost the sword, to which Mad Jack defends himself by claiming he's the reason why they are alive. As he attacks Jack, Jack calmly controls his anger and finally sees through his rage, vanquishing Mad Jack for good as the latter begs "You need me". After battling his inner demon, Jack finally reclaims the Katana.