The firing's stopped. I need a new host body -- I've almost burned this one out. But I daren't take one now. For the few seconds I'll need to possess any of these X-Men, I'll be too vulnerable. The risk is too great. Better to run, and survive.
Kevin MacTaggert is the son of mutant expert Moira MacTaggert, and politician Joe MacTaggert. The boy was a mutant with the ability to warp realities. These powers burn out his body, leaving him in need of new host bodies to sustain himself. Kevin's only known weaknesses are metal and his need for fresh host bodies. Due to the unstable nature of his powers, Moira confined Kevin to a maximum security cell inside her mutant research facility located on Muir Island.
During a battle between Magneto and the X-Men, the facility's security was compromised and Kevin's cell was damaged. This caused what remained of his body to burn out and required him to look for a new host. When a man came to destroy Moira's laboratory, Kevin possessed him and hid until he could escape the island. He later tried to possess Phoenix and Polaris, but both attempts failed. Kevin managed to escape the island in one of Multiple Man's duplicate bodies and began a quest to find and possess his father's body.
Even with the timely intervention of the X-Men, Kevin still managed to possess the body of his father, Joe, leaving a trail of discarded host bodies in his wake. During the battle, however, Joe's body also burned out leaving Kevin (now calling himself Proteus) vulnerable in his energy form. The X-Man Colossus used his metal form to slay the boy. His energies were disrupted and scattered all over Scotland. All thought Kevin to be dead.
Sometime later, a woman by the name of Erika Benson (also known as Harness) was hired by AIM to use her son, Gilbert Benson who she called Piecemeal, to collect the energies of Proteus for their own use. Gilbert had the ability to absorb the energies that were once Proteus, and his mother intended to collect it all. Her mission was almost thwarted by the New Mutants, New Warriors as well as Moira MacTaggert and a number of her mutants from Muir Island. Their intervention failed, and Gilbert's body exploded, resulting in a fusion entity that was both Gilbert Benson and Kevin MacTaggert.
This new Proteus used his powers to reshape Edinburgh, Scotland to his own whim. The arrival of X-Factor brought a philosophical tone to the battle, as the heroes questioned the ethics of killing Proteus or trying to help them. In the end, Proteus realized that reality was not compatible with him and "killed" himself.
Kevin was not dead, as his body is a lattice of energy he just entered a low-energy resting state, both dead and alive at the same time. When Selene and Eli Bard used the trans-mode virus to resurrect the dead a piece of Proteus was carried with them, creating a link he could use to restore himself. Using the mutant Destiny he distorted her visions to lure Blindfold to Muir Island. Turning her into a psychic magnet, she collected up his fragments. Getting the final piece from Destiny, Blindfold became Proteus' new host. He proceeded to attack the X-Men with newly expanded powers, possessing Blindfold, Destiny, Nightcrawler, Psylocke, Husk, and Trance, turning the iron in Magneto's blood into lead, and knocking Colossus and destroying the Blackbird's cockpit. Right as he was about to kill Rogue Magneto launched both himself and Proteus into orbit.
With prolonged proximity to Proteus, Magneto was able analyze his energy matrix while simultaneously using his electromagnetic powers to shield himself from Proteus' possession. After enough study, Magneto was able to change the part of Proteus that allowed him to possess Blindfold, and kick him out of her body. Unstable, Proteus dissipated. Magneto, however, warned that Proteus would be back.
Energy Form: Kevin is a psionic being composed of energy that has a vaguely human form with no physical characteristics.
Possession: Kevin requires human hosts to sustain himself and his power increases with each new victim. He can possess several bodies at one time. These bodies eventually "burn out" and become nothing more than animated corpses before he disposes of them.
Reality Warp: He can manipulate energy and matter in a limited area around himself with a thought.
Telepathic Immunity: He is invisible to telepathic scans and most detection devices.
Dimensional Teleportation: Kevin can shift between realities.