Nothing is known of Jacob Greene's early life. During World War Two he flew secret missions for the Allies in Europe. In 1961 he was chosen to be on the crew of the Artemis mission - the first manned trip to the Moon.
On the way, the ship was intercepted by a rift in spacetime - they entered The Bleed, the interdimensional buffer zone between universes. They emerged in orbit around an alternate Earth, one much more advanced and paranoid than their own.
The residents of this Earth saw Greene's Earth as a potential threat, so they made a bargain with the crew of the Artemis. The four astronauts would be given metahuman abilities, in return for preparing their Earth for eventual invasion and destruction. The Artemis crew agreed, and returned to their world with fantastic new powers.
For Greene, however, the powers came with a price - he was mutated into a monster of horrible visage. Where the other members of The Four could mix with regular people, Greene had been cut off from all humanity by his grotesque appearance.
He worked for The Four as a killing machine, and was nearly unstoppable. His kills lacked delicacy or style, but they were efficient and unquestionably final.
Greene was captured by Elijah Snow, the leader of *Planetary* and was stranded in deep space, where it is presume he has survived. Whether he will be returned to the Earth at some point is unknown.