Forge

Forge does not think about machines. He looks at a pile of parts and simply knows what they want to become, a mutant gift for invention so intuitive that he builds working technology decades ahead of the field without drafting a schematic first. He is also a trained Cheyenne mystic, which makes him one of very few Marvel characters fluent in both circuit boards and spirit work. That combination is exactly why the X-Men keep him around: he is the man who maintains the Danger Room, and also the man who knows what to do when the thing loose in the Danger Room is a demon.

The cost is written on his body. In Vietnam, with his unit dying around him, he opened a door and let something through, and it devoured the battlefield. He came home missing a hand and a leg and carrying the knowledge that he had personally summoned the Adversary into the world. He built his own replacements. Later he built a mutant-power neutraliser on a government contract and watched it hit Storm instead of its target, a mistake that cost her everything and then somehow became a love story. Invention colliding with consequence, every time.

Claremont and John Romita Jr. introduced Forge in 1984 as the man who accidentally broke Storm, which is an unusual entry point for a love interest and a superb one. He then carried the Adversary through Fall of the Mutants, sitting at the centre of the storyline that killed and resurrected the entire team. From there Marvel used him as the X-books' standing engineering department, handling Danger Room upgrades, X-Factor hardware and Sentinel countermeasures, and the Krakoa era slotted him onto X-Force as its armourer. He is the rare supporting character who stayed useful for forty years precisely because the plot always needs something built.