Rex Mason was an adventurer hired by Simon Stagg to retrieve a rare Egyptian artifact. After it was learned that Rex had been dating Stagg's daughter, Stagg began making a plot to kill him.
In an Egyptian pyramid, Rex was knocked unconscious by one of Stagg's bodyguards, and exposed to a radioactive meteorite. This transformed Rex into Metamorpho, the Element Man. Metamorpho is one of many metamorphae, a race created by the Egyptian sun god Ra. With his abilities, Metamorpho can transform into any element, in solid, liquid and gas form.
Rex would later be recruited by Batman alongside with Katana, Black Lightning, Halo, and Geo-Force to be a founding member of Batman's new super-hero team, The Outsiders. The Outsiders were Batman's personal Justice League who had the job of going on missions which would be possibly controversial in the eyes of the public which the Justice League or Bat-Family could not go on.