Neil Sinclair was a soldier from the US Army in the universe of Flashpoint. He volunteered to became a subject of experimentation in the Project M division called Project Superman, under the command of Sam Lane. The intention was to create supersoldiers capable of repelling the forces of the Atlanteans or the Amazons were they to threaten the world. For that, Sinclair received DNA grafts taken from the being called Project Six aka Doomsday, making SInclair the Subject Zero of these experiments. These grafts progressively transformed Sinclair into a hulkish being of uncontrollable strength and an ample array of powers. However, same as his powers grew, so did it his uncontrolled rage and paranoia. After his first mission on the field ended in a massacre, Subject Zero was put into prison, but he still was capable of seeing everything happening around him.
When Kal-El arrived to earth and was capture by Project M, Sinclair manipulated the boy for years, unknown to anyone in the project. He eventually convinced Kal to free him and he started to destroy the Project M base. However when he tried to kill a young Lois Lane, Kal opposed him and he distracted Subject Zero enough for the General Lane to send Sinclair and himself to the Phantom Zone.
When Barry Allen and his motley crew of allies freed Kal, they caused the source of energy keeping Sinclair in the Phantom Zone to be disconnected. Now returned to the real world, Sinclair killed Sam Lane and destroyed what was left of the base. He also absorbed more of Doomsday, making his powers grow even more, acquiring the ability of flying.
With this new ability, Sinclair traveled to England, where Kal had went to save Lois Lane from the Amazons. Sinclair fought against Kal, and the Amazons troops but soon it was clear than he lacked the control of the powers he flaunted so much and Kal killed him when he lost the strength he has just gained. However, his explosive demise also caused the death of Lois Lane, who with her last breath convinced the resilient Kal to save the people of the country.