Zero-real name David Oh-was sort of the mastermind behind many of the events in the Metal Gear saga. Although the story really starts with the Philosophers-a group of rich citizens dedicated to uniting the world-Zero later recreates that organization as the Patriots, and sets many later events into motion, including Big Boss’s fall into darkness, and the creation of Solid Snake and his brothers.
Zero’s story pretty much begins in World War II, as he was a member of the British commando team Layforce, and worked closely with the Joy, or the Boss, the special forces commander who led the Cobra Unit and-in the universe of Metal Gear-were instrumental in helping to win World War II. She also later trained John/Jack, who would later be dubbed Naked Snake. He later joined the British secret service MI6, where he aided in the defection of Russian scientist Sokolov.
However, in order to ease tensions after the Cuban Missile Crisis, Sokolov is returned to the USSR. Eventually, Zero forms his own special forces group-FOX-and one of their first missions-code named the “Virtuous Mission” is to rescue Sokolov, who is rumored to be developing a weapon. Zero sends FOX operative Naked Snake on the mission to rescue Sokolov. However, things go eschew when it appears the Boss has defected, and one of the nukes she brought as a sign of goodwill toward rogue Russian colonel Volgin is used to destroy Sokolov’s facility.
Snake is deployed again by Zero, for Operation Snake Eater-but if they screw this one up, it’s possible they will be executed, as blame for the first mission’s failure is based. Snake’s mission is to kill his mentor, destroy Sokolov’s weapon the Shagohod and also once again attempt to rescue Sokolov. He advises Snake several times during the mission, and also displays a peculiar sense of humor and concern over Snake. We also learn he’s a big James Bond fan.
After Snake accomplishes two of those objectives, it turns out the Boss was on an undercover mission all along to steal a microfilm from Volgin.
After an appearance of debatable canon in Portable ops where he is suspected of staging a rebellion due to elements of the Fox Unit going rogue, Zero and Big Boss, along with the other members of operation Snake Eater form the Patriots. Before she died, the Boss related that she wanted to make the world whole. The Patriots/Cipher were formed as an attempt to realize that dream. However, Zero and Big Boss’s views on the subject differed. Zero felt the world could be controlled by information, Big Boss wanted a world where soldiers could be free to fight for what they believed in. Zero also begins to lose his grip on reality, becoming more greedy and paranoid.
Fearing he might lose the support of his friend, Zero-now calling himself “Cipher” as well-commisions Doctor Clark to clone Big Boss, in a project known as Les Infant Terribles, which results in the creation of three sons of Big Boss-David (Solid Snake) Eli (Liquid Snake) and George (Solidus Snake). This leads to the final rift between the two, and Big Boss flees to Costa Rica to form his mercenary group, Militaries san frontieras with Kaz Miller.
Zero works out a deal with Miller, which in turn helps Miller help to expand MSF, although Big Boss is unaware of Zero’s involvement. Zero also covertly aids the Peace Walker project, and inserts a spy into Big Boss’s ranks-the girl known as Paz, who posed as a college student wishing for peace, but was actually a Cipher agent.
After the Peace Walker incident, Paz took control of MSF’s Metal Gear ZEKE, and threatened to use it’s nuke and frame MSF is Big Boss did not rejoin Cipher.
Although Paz is defeated, things start to go out of control for Zero. His second-in-command, Skull Face, captures Paz and uses her as a complicated trap, which ends with the destruction of MSF, Paz’s death, and Big Boss falling into a coma from injuries after a helicopter crash. Learning his location from Paz,Skull Face also poisons Zero with an early strain of a parasite, causing him to begin to fall into a vegetative state, dealing a sort of coup to Zero.
Zero, with events falling out of his control, starts to take action to defeat his former XO and protect his former friend-despite their past differences, as well as to ensure his own legacy, and the Boss’s. He hides Big Boss in a hospital in Cyprus, and also begins to create a decoy/body double for Big Boss-Venom Snake-to protect the real one, with the help of Revolver Ocelot. (It’s Venom who eventually defeats Skull Face). He also has Donald Anderson-the former SIGINT from Operation Snake Eater-develop the Patriots AI system to ensure that his version of the Boss’s dream is realized. Finally, he collapses into a vegatative state sometime in the early 80’s, although many assume he is still in control of the Patriots. His location is hidden.
Eventually, the Patriots AI system begins to take more and more control over the world and information, especially during the time of Solid Snake. However, it begins to deviate from the will of the Boss and Zero. When it’s eventually defeated due to the actions of Solid Snake, Otacon, Sunny Emmerich, Revolver Ocelot and Naomi Hunter, not only does Big Boss awake from a near death state (sent into that by Solid Snake nine years earlier), but Zero’s location is revealed. Big Boss tracks down his old ‘frenemy’, and the two meet Solid Snake at Arlington cemetery.
There, Big Boss cuts off Zero’s life support (and he dies as well), ending the cycle of evil they created by misinterpreting the will of the Boss.