Kevin MacTaggert is the son of Dr. Moira MacTaggert and her husband, Joseph MacTaggert, who forced Moira into an unhappy marriage. Kevin was conceived after his mother, Moira, was severely beaten and raped by Joseph. After this traumatic experience, Moira left Joseph. She did not notify him that she was pregnant with his child. Moira and Kevin lived at her Mutant Research Centre on Muir Island, off the coast of Scotland. Kevin eventually began manifesting his mutant abilities and became a danger to everyone around him. His abilities gave him an uncontrollable hunger for energy. To protect herself and others, Moira was forced to confine Kevin. To keep the truth about her son secret, she began to describe him to others only as Mutant X. For years, Kevin remained inside his cell, sustained by isoteric energy fields that kept his body from burning itself out, until one day, after a battle between Magneto and the X-Men, his cell was damaged and Kevin was able to escape when he took over the body of Angus MacWhirther. During his escape, Kevin came into contact with Phoenix, Polaris, and Multiple Man. While he was able to defeat Polaris and Multiple Man, he barely escaped with his life when he confronted Phoenix. Without the isosteric energy fields of his cell to sustain him, Kevin began to burn his body out. The only solution he could find was to possess human host bodies, one after another, including one of Multiple Man's duplicates, Ferdie Duncan, an unnamed policeman, and Jennie Banks. Kevin eventually encountered the X-Men and proceeded to rename himself "Proteus," after the Greek god. Proteus eventually went to Edinburgh, to possess his father, Joseph. After possessing his father, Proteus made a last stand against the X-Men. Joseph MacTaggert's body was destroyed in the battle. Before Proteus could take another host, Colossus punched Proteus in his energy form. Due to Proteus' intolerance for metal, he was unable to maintain his energy form. His energy was therefore dispersed across the world, and the X-Men and Moira assumed that he had been killed. Some time after his death, Moira MacTaggert thought about cloning Kevin, but she was discouraged successfully by her boyfriend Sean Cassidy. Several years later, A.I.M. attempted to recreate Proteus. The organization used a woman named Harness and her mutant son, Piecemeal, to absorb all of the dispersed energy of Proteus. Piecemeal and Harness encountered the New Mutants, but escaped them. As the boy went about absorbing the energy, his body grew too large for his system to handle. Eventually, what was left of Proteus' consciousness and Piecemeal merged as one being after Piecemeal absorbed all the extant Proteus energy. The combined efforts of the New Warriors, the New Mutants, the Muir Islanders, and the original X-Factor wasn't enough against this new creature. When the amalgamated being decided that it would not find happiness, it decided to disperse once more, effectively committing "suicide".
House of M/Exiles
When the Scarlet Witch reshapes Earth-616, Proteus is resurrected as the serial killer "Mutant X," who steals bodies across two continents and is hunted down by Moira MacTaggert, who is framed by the government for Proteus's murders. He encounters the Exiles and actually manages to escape the House of M by stealing data from the Panoptichron. Notably, when he leaves his home reality he inhabits bodies of superhumans from "approximation" realities or Exiles members only. He has inhabited the bodies of an unnamed mutant, of Blunderbuss, and of Angel Salvadore in House of M; Mimic and Morph in Exiles; Justice from the New Universe; and the Hulk from the Marvel 2099 universe. None of these bodies lasts for long, with the exception of Angel Salvadore. Other reality manipulators, such as Longshot may be immune to Proteus' manipulations. Mutant X's current body, Morph from Exiles, also does not seem to deteriorate while Proteus inhabits it. Proteus states that he could not stop thinking about Blink. Whether it was because he still had some of Mimic's memories or because he actually felt something for her is not yet clear. He states he was tired of fighting and that he just wants to talk, but before he can say anything, Longshot shields Blink, allowing her to teleport to safety. It is revealed by Proteus himself that his mind is able to recall all memories and feelings of his previous hosts. Blink tricks Proteus into wearing a portable Behavior Modification System (from the Squadron Supreme's world), which she teleported into the crown he was wearing. The device brainwashes him into believing he is actually Morph and leaves him able to recall only Morph's memories prior to his possession. Unaware of his true identity, he begins behaving exactly like Morph and a member of the Exiles, much to the discomfort of the other Exiles. With Proteus trapped and believing he is Morph, he remained an Exile to continue fixing damaged realities. Considering Morph's body does not burn out like other hosts, Morph's consciousness is still active beneath Proteus. Also, Proteus is immune to metal while in Morph's body, since wearing a metal tiara during the "Heroes Reborn" world did not kill him. However, concerns about some discrepancies in "Morph's" behavior forced his teammates to plan regular brainwashings with the same device, and, eventually, put him in stasis whenever his behavior again became abnormal. However, that device was destroyed when Psylocke and Sabretooth fought so intensely that they shook the Crystal Palace, causing a bookcase to fall on Morph's head, shattering the device. With the device shattered, it was only a matter of time before Proteus would re-emerge. Recently, during a confrontation where Proteus reawakened, he found himself lacking in power to defeat the adversary. About to be crushed, Proteus shouted aloud his desire to stay alive. In a vision, he saw a figure, almost identical to his own true energy form, telling him to take its hand, and he would survive. Upon doing so, Proteus found himself full of even greater power, using it to defeat his god-like enemy. Afterwards, it was revealed that this being was the personality and soul of the true Morph, having been in limbo, gaining strength within Proteus and his own body, who had before only been able to speak a few sentences through Proteus' control. Revealing to Proteus that he had the ability to eject him from his body, Morph gave Proteus the chance to work together and share his body and their powers, in order to do more good, something which Morph had discovered Proteus desired deep inside. Proteus accepted, and the two now work in harmony, better than either could be alone. However, soon after, when the New Exiles became immersed within the Crystal Palace, Proteus was absorbed in Morph's place, freeing Morph once and for all.
By some yet unknown means Proteus appeared inside Destiny when she was resurrected by Selene. When Destiny tried telepathically contacting her foster-daughter Rogue, she instead found Blindfold. There Proteus possessed the young girl and used her to send some of the X-Men to Muir Island where he reveals himself to Rogue. Proteus easily takes down the team, punching Colossus halfway across the island, turning Magneto's blood to lead and taking mental possession of Nightcrawler, Husk, Trance and Psylocke, pitting them againt Rogue. Magneto uses the last of his strength to launch himself and Proteus into the upper atmosphere, buying himself some time to fully understand the energies that make up Proteus' form. When Magneto establishes he can manipulate the energies, he uses his powers to disrupt them, dispersing Proteus' form. He does confirm though it will only be a matter of time before Proteus can reintegrate himself.
Powers and Abilities
Reality Warping: Proteus is a mutant who possesses the vast psionic ability to manipulate and alter reality. Originally Proteus could not use his reality warping powers against another person unless he made visual contact with his victim. However, he apparently did not suffer from this limitation with regard to inanimate objects, since he was able to extend a "reality warp" over much or the entire city of Edinburgh.
Telepathy: Proteus also possesses telepathic abilities. Some of the X-Men have theorized in the past that Proteus merely creates illusions in people's minds that he manipulates reality, but the preponderance of evidence is that Proteus's reality-warping powers are indeed real. So the true extent of his telepathic powers are still unknown.
Energized Form: Proteus exists in a pure energy form.
Physical Possession: Can take full possession of humanoid bodies.
Proteus has no physical strength save for that which he gets from his host bodies.
Power Burn Out: After Proteus' powers "burned out" his original body, he existed in a state of pure psionic energy. He could take possession of a human body as a host, but his energies rapidly "burned out" the body, killing it. His energy form would rapidly disperse unless he took possession of a new host.
Metal Vulnerability: He is also vulnerable to organic metal such as Colossus' body. Dr. MacTaggert once kept Proteus imprisoned within a metal cell, but it has not been explained why Proteus did not simply use his reality-warping powers against her to escape; perhaps the cell's special energy fields somehow prevented him from escaping. After Proteus and Piecemeal merged, the resulting energy being was no longer vulnerable to metal. Moreover, the amalgamated being apparently no longer needed host bodies in order to survive, nor did it have to see a living victim in order to use his reality-warping powers upon him.