Alias of an alien mercenary agent (Xviar) sent to Earth from "Homeworld" to observe Kal-El's life and to prepare the way for using Superman as an instrument for the planet's destruction. As part of this end, he arrives on Earth as Kal lands and follows him for 30 years, including moving into the apartment next to Clark Kent. (S No. 258/2, Nov 1972: "Who is Mr. X?")
Manipulating Superman's perceptions of his powers by treating his clothing with a chemical that blocks yellow sun radiation, Xavier also uses instruments in the apartment to communicate telepathically with Superman and to transform him into an explosive living power. Space jewels in Clark's possession are employed to summon nine of the most fearsome foes Superman has ever faced to Earth, so that Superman's fights with them can feed the process that makes him the "human bomb" that could destroy the planet.
Superman finally figures out the scheme, one part of a contract "Homeword" has to clear the Earth from an intergalalctic teleportation route, and he also foils Xavier's attempt to escape the solar system by posing as one of the foes Superman has defeated (Amalak). (S No.s 296-299, Jan-May 1976: "Who Took the Super Out of Superman?"; "Clark Kent Forever...Superman Never!"; "Clark Kent, Get Out of My Life!"; "The Double-or-Nothing Life of Superman!")