While he hasn't been around the Marvel Cinematic Universe for long, John Walker has made a huge impact as the new Captain America of the Marvel Cinematic Universe.  Over several episodes, he has been rubbing the titular heroes of The Falcon and the Winter Soldier the wrong way with his overly aggressive and zealous brand of patriotism. These are traits that the John Walker of the MCU has in common with his comic book counterpart, but there are plenty of aspects to his character that are unlikely to make the leap from the comic page. Most significant of all is the U.S.Agent's unique prosthesis, courtesy of a familiar symbiote from an unfamiliar world.

The John Walker of the comics had a long and painful career as a hero, villain and everything in between. During the events of 2010's Siege, the U.S.Agent found himself drafted onto the roster of the Mighty Avengers alongside the likes of Amadeus Cho. This version of the team would find themselves in conflict with the Thunderbolts, and their confrontation in Thunderbolts #142, by Jeff Parker and Wellington Alves, changed John Walker forever. Walker grappled with fellow Captain America wannabe Frank Simpson over the mythical spear of Odin, and the fight resulted in Walker losing his arm and a leg.

Not willing to let his injuries sideline him permanently, John Walker found a solid retirement gig as the warden of the superhuman prison The Raft. As fate would have it, his duties as warden would put him in a position to return to heroics when he crossed paths with a Venom from an entirely different universe.

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In 2012's Dark Avengers #183, by Jeff Parker and Neil Edwards, the titular team faced off against the Thunderbolts while attempting to escape from Walker's custody while being escorted to their cozy new prison cells. An attack from Toxie Doxie during the jailbreak sent the transport hurtling through a portal created by Man-Thing and John Walker alone and unconscious on another Earth in another universe. This world inside a pocket universe had been left ravaged by A.I.M., who had spent decades whittling down its population with inhumane experiments.

Among these experiments was a piece of the Venom symbiote which had been unsuccessfully used to power the next generation of Iron Man armor. These experiments left the symbiote with effectively no consciousness of its own. At the same time, it made it the perfect blank slate from which to work, and Toxie Doxie was quick to fashion it into something seriously shocking. After so much time spent knocked out from the crash, John Walker finally awoke to discover that his missing limbs had been replaced.

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This wasn't a one-off storyline in the broader scope of things, but rather a permanent return to form for John Walker, though not one that is mentioned very often. The symbiote that John was bonded to was absolutely the other world's Venom, but after having been effectively lobotomized, the nuances of what a symbiote is were lost. It's almost surprising that it was able to replace his missing limbs in the first place, considering there is no mental or cognitive bond to be formed, and as such John is in no way connected to a symbiote Hive nor is he considered a proper host to his symbiote. Still, this often overlooked aspect of his character is one that has held true for nearly a decade and has informed just how colorful of a career John Walker has had since his own introduction 35 years ago.

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