Machine Man (Earth-616)

Machine Man, also known as X-51 or Aaron Stack, is a superhero appearing in the Marvel Comics Universe. He is a synthezoid, or "synthetic humanoid" robot.

History
As attempts to replicate the Super Soldier experiment that created Captain America repeatedly met with failure, an alternative military project, drawing inspiration from the synthezoid Avenger Vision, attempted to create an army of synthezoid super-soldiers. The lead scientist, Dr. Abel Stack, felt that his creations had potential to be more than mere killing machines; he felt that his creations could function like humans if they were raised like humans. To prove this theory, Stack brought one of them, X-51, to his home and raised him like a son. While the other synthezoids in the project went rampant and had to be put down, X-51 did not. Knowing that his superiors had placed an explosive killswitch inside X-51, Stack attempted to remove it. Though he was successful, the bomb had a higher yield than he thought, and it had already been primed by remote. Stack sacrificed himself to shield X-51 from the blast, and vowed to honor his "father's" sacrifice by helping to bring about his vision of peaceful co-existence between organic and synthetic life.

Hunted by creator's former bosses, X-51 fled to New York. He found that machines were mistrusted and feared, but that he could easily pass as a human. He befriended psychiatrist Dr. Peter Spaulding and mechanic Wilbur Garvin, who helped him set up a human identity as Aaron Stack, a claims investigator for Delmar Insurance. To show humans that his kind need not be feared, he becomes the superhero Machine Man. He gains an arch-enemy in the form of Madame Menace, the self-described Armorer of the Underworld. Seeking allies within the superhero community, he befriends the Fantastic Four and Alpha Flight, and falls in love with Vision's "sister" Jocasta. He seeks out Iron Man, believing him to be a robot, instead finding a very much human Tony Stark, having relapsed into alcoholism due to the machinations of Obadiah Stane. In a drunken state of paranoia, Iron Man attacks Machine Man, who finds himself having to protect bystanders from the Armored Avenger's drunken recklessness. He eventually incapacitates Iron Man, concluding that Stark has something seriously wrong with him.

After Jocasta appears to perish in a battle with Ultron, the Super-Adaptoid offers to rebuild her if Machine Man joins Heavy Metal, a team of androids he is assembling to fight the Avengers. Machine Man man agrees, and they attack the Avengers Hydrobase. When it becomes evident that the Super-Adaptoid was lying and he was really seeking to steal a Cosmic Cube that Namor was safeguarding, Machine Man switched sides and helped the Avengers. He fought alongside them again in a battle against Terminus the Destroyer; his bravery in that battle prompted him to named a reserve member of their West Coast branch. He was eventually captured by the cyborg Bastion, who hoped to convert him into a new breed of Sentinel to hunt down mutants. Though he eventually broke free of the Sentinel programming, being upgraded with Sentinel tech in the process, his fellow Avengers were leery of the danger he might pose to their mutant members if the programming reasserted itself.

After performing some counter-terrorist operations for S.H.I.E.L.D., Machine Man was eventually reunited with a resurrected Jocasta in their Alternate Reality Monitoring and Operational Response Agency. There they had numerous adventures on alternate universes, battling Death's Head and a villainous Iron Man from Earth-8410. They end up going rogue during a zombie plague, where they had to illegally steal several samples in order to find a cure.

After returning to Earth-616, Machine Man and Jocasta moved in together, and Jocasta began working for Stark Unlimited. They joined forces with Iron Man against plots by Controller and Ultron to manipulate Stark Unlimited's new cyberspace software, but Ultron's recent actions increased tensions between organic and synthetic life. In light of the increasing oppression faced by artificial beings, Machine Man started to incite an uprising in the A.I. population to fight for their rights. He soon discovered that the forces of A.I. oppression were being led by his old adversary, Madame Menace, and Tony Stark's long-lost brother, Arno, who had recently claimed the mantle of Iron Man for himself. Machine Man was pleasantly surprised when Tony himself joined the rebellion, using an earlier suit of armor he dubbed Mark One. As Tony and Arno battled each other for the mantle of Iron Man, Machine Man found himself battling earlier versions of himself, and his intended upgraded successor, X-52. After Arno was defeated, Machine Man and Jocasta turned their resistance group into a grass roots civil rights organization.