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AKA The Monster. First appeared in issue #21 (and previously, obscured in a Cole's flashback sequence in issue #10). Jack' life was ruled by a heroin addiction while he worked occasional odd jobs as a bouncer in Boston. He was given an attaché by Graves that was (apparently) the only one containing a picture of its recipient, essentially prompting him to commit suicide. After receiving the attaché, Jack instead carried on with his life of addiction and crime.
Jack and his friend Mikey later made their way toward Atlantic City, where they ran across a business run by Mikey's cousin: a zoo full of wild animals that, for a fee, could be "hunted" by mafiosi for thrills. When a caged tiger brought back vague memories of his previous life, it is revealed that Jack was a Minuteman at the time of the events of Atlantic City, and that after being deactivated, he had merely descended into a pattern of substance abuse fueled by depressive self-loathing that had been established much earlier in his life.
After the personal revelation, Jack parted ways with Mikey, kicked his heroin habit once and for all, and took up bareknuckle street boxing in Atlantic City. Graves, openly scornful of the waste that Jack has made of his life, seemed reluctant to reactivate him before he could fully overcome his erratic and dangerous tendencies. Lono then fully reactivated Jack, though whether this was under direct instruction from Shepherd, Graves, or by his own inclination is unknown.
After working for Lono for a while and traveling with Loop and Victor Ray, Jack bolted on his own after receiving the La Morte dil Cesar painting from Cole. He was later found with Echo Memoria by Loop and Cole. Jack then joined the surviving Minutemen as they went to confront Graves in the series' finale. In issue #100, Jack was confronted by Crete, who attempted to stop him from entering the Medici household. The two fought and ultimately tumbled into the alligator pit during their struggle, where it is strongly suggested that both were eaten. However, given the nature of the story and Jack's massive strength, his survival is not out of the question, leaving his future unknown.
Jack is physically the largest of all the Minutemen which makes him able to inflict and sustain large amounts of pain. Lono himself stated that Jack was the one most capable of taking out any of the other Minutemen due to his massive size and skills as a fighter. This, paired with the ongoing struggle with his crippling heroin addiction, is the most likely inspiration for his nickname: "The Monster".
Before the series' final issue, Jack reveals that he's developed a distaste for killing after having lived the life of a Minuteman. He nevertheless seemingly dies in the finale while engaged in hand-to-hand combat with the equally imposing Crete. This lends credence to Graves' suggestion that he is his own worst enemy, as Jack remains married to his self-destructive tendencies to the end.
Jack was reactivated in issue #68/69, whilst facing Lono is an underground boxing match. Lono arrived for the fight with the word Croatoa freshly tattooed across his stomach, revealing it to Jack when he removed his shirt for the fight.