Simon Stagg is the man responsible for creating Metamorpho.
Simon Stagg attempted to create the ultimate miner—'Metamorpho'. Stagg used his Stagg Enterprises Chemical Division to create a formula capable of turning a human into a shape-shifting soldier. He intended to then sell this being to companies working in dangerous conditions, to lower their insurance costs and make money for himself.
The mutagen was delivered to Stagg from his laboratories by his bodyguard, Java. In transit, however, a test tube of the chemical fell out of the briefcase carrying the samples, which melted the axle of the train, causing a train wreck which Green Lantern failed to avert. Stagg blamed Java for the loss of the chemical sample—which Batman picked up upon investigating the crash site. Rex Mason, an employee of Stagg's, then discovered the illegal mutagen project and threatened to blow the whistle on Stagg.
Visiting his daughter, Sapphire, later that evening, Stagg learned that she was engaged to Mason. Enraged by this, Stagg decided to test the chemical on Mason by tricking him into the laboratory as he was cleaning out his desk. The mutagen turned Mason into a shape shifting mutant, with a distinctly non-human appearance. Enraged by this, as well as confused by the changes to his body, Mason, dubbing himself Metamorpho, decided to carry out revenge against Stagg, until Stagg tricked him into believing that Sapphire was dating another man—John Stewart. Metamorpho, mad with anger, turned his sights on destroying Stewart while Stagg enjoyed the show, taping every minute of the conflict in order to prove to his clients what Metamorpho could do.
However, when Metamorpho learned that Sapphire wasn't cheating on him, he went back after Stagg. Upon arriving at Stagg Enterprises, Metamorpho confronted Stagg about why he was chosen to be transformed. After Metamorpho mentioned Sapphire, Stagg told him he would never let him touch his daughter again. Metamorpho was then frozen by Java. Back at Stagg Labs, Stagg attempted to create a new version of the formula, until Metamorpho—who had unfrozen and escaped—intervened. In the tussle, the system Stagg was linked to short-circuited, thus Stagg's mind was transferred into the formula, which became a giant, semi-sentient slime creature. His mind trapped in the monster's form, Stagg's unconscious body was found by the Justice League.
With part of his mind trapped in the beast, Stagg rampaged through Metropolis trying to find his daughter. After a battle with the Justice League, Stagg found Sapphire at her apartment and took her before battling the Justice League again as well as Metamorpho. Easily defeating them, Stagg then took his daughter and climbed the tallest building he could find. At the last moment, when Batman discovered that the way to stop Stagg was by forming a polypeptide that would dissolve the creature. Metamorpho demanded that the creature let Sapphire go, but taking on Stagg's face, the creature refused. Metamorpho morphed into the chemical and dived inside the creature's monstrous form. As a result, the creature was completely destroyed and its mind returned to Stagg, while Metamorpho regenerated and came back to Sapphire.