The X Man (a.k.a. The Phantom or Mr. X) was the nickname given to an extraterrestrial humanoid whose spaceship crashed on Earth in 1957. His body is invisible to the human eye, although it becomes visible under ultraviolet light. He wears a protective suit with a breathing apparatus, the latter which contains a strange mix of gases that isn't breathable for humans, and which includes an 11% percentage of methane. The suit is also dangerously radioactive, to the point that a human being would never survive wearing it. Other features of the suit include that it is made of an unknown, but apparently metallic material, able to resist unscathed any kind of knife, acid or fire; and that it is strongly magnetic, and interferes with nearby communication devices.
Even though he is able to survive in Earth atmosphere without the suit for some time, the X Man cannot last long without his suit and helmet, and will only take them out in situations where he desperately needs to escape without being seen. Initially believed to be hostile due to having killed two people, being present in an explosion site and interfering with communication systems; it was later revealed that the alien was merely a scared and desperate castaway, lost on a world that was strange and menacing to him. The deaths he caused were on self defense, and he didn't cause the explosion but was drawn to it due to the gases released which were closer to the atmosphere he was accustomed. Once meeting Dr. Barbara Ramsey in her lab to retrieve his lost helmet, the alien attempted to communicate with a Morse-like code, but wasn't successful.
This mysterious being has a highly unusual physiology compared to Humans and it is speculated that his body may have been silicon-based. Although he can communicate verbally, it is in a frequency that the human ear cannot detect (although dogs can). His body also becomes briefly visible after death (revealed to be almost identical to a Human, except for the bald, enlarged cranium), after which it sublimates to nothingness. The meaning of the code the alien was trying to communicate will likely never be known.