Who: X-51, or Aaron Stack, was the only survivor of an experimental government program, who first appeared in the 2001: A Space Odyssey* comic book #8 by Jack "King" Kirby. (*Yes, Marvel really did try to pimp out any licensed property they could in the 1970s. There was never a Barry Lyndon comic, however.) In his own series, X-51 went on to learn about being human. Kirby's striking design - I always loved the telescoping arms and legs, such a 1970s idea of "futuristic" - was one of his great later-career works, even if the writing was sometimes a little simplistic.
Read This: Kirby's Machine Man comics are annoyingly not available in collected editions, although pretty much everything else the man did is. (There's some conflict with MM's first appearance in the "2001" comic which Marvel no longer has the rights to.) But you must seek out the gorgeously drawn Machine Man series from the 1980s, and for the sarcastic twist on the character, Ellis' sneeringly satirical "NextWave" series. Machine Man has also been a guest-star everywhere from Hulk to The Avengers over the years and for some reason whenever old telescope-arms pops up, it cheers me up. Long live X-51.