While his real name is never mentioned, it is known that Sundowner was formerly an average student who joined the U.S. Military out of school. He led a spotty career, always effective and skilled but investigated several times for alleged war crimes violations. Despite never being charged he left the military and became a mercenary, earning infamy during the days of the S.O.P. system. It was during this time that he earned the moniker "Sundowner", referring to both the deadly Southern California wind anomaly and the scenes of bloodshed he left in his wake, said to resemble a sunset. His military career was brought to an end by an improvised explosive device, and Sundowner was left crippled and bedridden until the fall of S.O.P. and the advent of widespread cyborg technology, which he made use of to restore his mobility and return to the battlefield more dangerous than ever.
Even going so far as to call himself a Warmonger, Sundowner had a sadistic streak and an obsessive love of combat and warfare. As the unofficial leader of Deserado Enforcement LLC, he and the rest of the Winds of Destruction carried out attacks and destabilized more than one region on the road to peace in order to ensure that there would continue to be violence they could profit from. Well armed and extremely well funded, they were a force to be reckoned with.
It was in Africa that one of Desperado's missions brought them into conflict with Maverick Security and Raiden, who were acting as bodyguards for Desperado's target, Prime Minister N'mani. Armed cyborgs boxed the prime minister's limo in and assaulted the military guards, drawing Raiden's attention while Sundowner and a modified, unmanned Metal Gear Ray disabled the Gemini cyborg guards and took the prime minister captive.
Telling Raiden that he intended to kill N'mani, he left the Ray to cover his escape. He was nonetheless pursued by Raiden and the military, but due to holding the Prime Minister as his hostage could not be attacked. Leaping onto a passing train where he reunited with "Jetstream" Samuel, the two planned to leave on an approaching aircraft but were confronted by Raiden, who had caught up with the train.
With the military unable to follow or interfere, Sundowner gave a quick speech to Raiden and the Prime Minister about the virtues and benefits or war, lamenting that for all it had done to improve mankind it was still viewed with disdain. With Sam holding Raiden off, Sundowner followed through on his promise to kill N'mani once his usefulness had run out and departed via aircraft.
While it's unknown exactly to what extent Sundowner was involved in the grand scheme of Senator Steven Armstrong, World Marshall was revealed to have been the source of the massive funding behind Desperado's operations, and the business operations of the two were so closely intertwined that they were almost the same entity. This included the operation of the Sears Program (named for former U.S. President George Sears A.K.A. Solidus Snake), as Sundowner and Armstrong were seen together making a progress check on the facility in Mexico and ordering the doctor in charge to dispose of all evidence and cease operations before they could be discovered. Raiden nonetheless managed to recover footage of the meeting and discover the nightmarish reality behind the Sears Program, a child-soldier training and brainwashing routine executed in virtual reality on the surgically removed and stored brains of abducted homeless children, to be implanted into cybernetic soldier bodies once they'd acclimated to their vicious training. All this was made possible due to the advanced technology developed by the Patriots, now widespread and available after the destruction of S.O.P.
When Raiden eventually attacked World Marshall's headquarters, Sundowner spoke to him over the intercom and told him to reach the top of the building so they could meet. Sundowner waited for Raiden to arrive at the storage and training room for the brains, and spoke about the plan World Marshall had and the futility of Raiden's actions, as another facility around the world could be up and running in no time at all even if Raiden were to succeed. Eventually, after taunting Raiden about the effectiveness of the Sears Program, he suggested they make their way outside to an area where they wouldn't damage the "merchandise".
Sundowner disposed of his coat before the battle, revealing six additional mechanical limbs sprouting from his back, each holding a large sheet of red-paneled reactive armor that would explode violently when struck, making him nearly impossible to attack head on. He also used his signature weapon "Bloodlust", a pair of modified high-frequency machetes that could combine into a deadly scissor-like contraption. To further stack the odds in his favor, Sundowner made use of unmanned Hammerhead helicopters and even Desperado soldiers once he'd lost the use of his armored shields.
In a last ditch effort to defeat Raiden in their battle, Sundowner fell from the heliport and grabbed a hammerhead, ordering it to fire upon the building and send Raiden falling to his death. The attempt was unsuccessful, and Raiden managed to reach and kill him in spectacular fashion. Despite mortal injuries, Sundowner managed to make one last call to Samuel to inform him that he'd get his wish of a showdown with Raiden after all.