Major Zero debuted as merely a replacement for Roy Campbell as mission COO in the Cold War era video game Metal Gear Solid 3: Snake Eater. It is later revealed that throughout the series, he is responsible for a great majority of its most important events for 50 years, from 1964 to 2014. He is introduced as a friend of Big Boss and of The Boss, and cofounded The Patriots to facilitate the Boss's dream alongside Big Boss, Sigint, Para-Medic, EVA, and Revolver Ocelot. However, as he grew more power-hungry he began to earn Big Boss's ire, and the two eventually parted ways.
Zero commissioned at least two known research and development projects that had a great impact on the Metal Gear storyline. The first was the Les Enfants Terrible project; the project to clone Big Boss, resulting in the births of Solid Snake, Liquid Snake and Solidus Snake. The second project was the Patriot A.I. Network responsible for the events of Metal Gear Solid 2: Sons of Liberty and Metal Gear Solid 4: Guns of the Patriots.
Metal Gear Solid 3: Snake Eater
Major Zero was the commanding officer of Naked Snake, the newest member of Zero's FOX unit. At the start of the mission, Zero immediately changed his codename to "Major Tom," after one of the escape tunnels dug in the film "The Great Escape." The mission was deemed a failure when The Boss defected to Russia shortly before its completion, recapturing Nikolai Stephanovich Sokolov before Snake could extract him, and allying herself with Colonel Volgin of the GRU, as well as bringing a gift of two Davy Crockett man-portable nuclear missiles - one of which Volgin fired at Sokolov's research facility. Major Zero would later revert his codename to its original state following the debriefing for the failed Virtuous Mission, after reviewing its film of origin and finding that the "Tom" tunnel had failed.
Operation Snake Eater
After the failure of the Virtuous Mission, both Zero and Snake were threatened with death by firing squad if their next mission were to fail. Their revised mission was to eliminate Colonel Volgin, the Shagohod (a precursor to the Metal Gears of the future), The Boss's Cobra Unit, and eventually, The Boss herself - following her acquisition of the Philosopher's Legacy and its transfer into CIA hands. The mission was a success, but for the Legacy having been split in two - one half going to the CIA, and the other acquired privately by Ocelot.
Metal Gear Solid: Portable Ops
San Hieronymo Takeover
Zero disbanded the FOX Unit in 1970, but was soon accused of treason because FOX had rebelled against the US under its new leader, Gene. Zero's name was later cleared thanks to the efforts of Big Boss, himself likewise under suspicion for possible involvement in the FOX Unit's actions in San Hieronymo.
In reality, however, Zero had sparked the incident himself, using Gene's threat of a Metal Gear-assisted nuclear strike on the Pentagon as a way to flush out and obtain the other half of the Philosopher's Legacy. With the Legacy united in his hands, Zero planned to create his own organization which would supplant the Philosophers and carry out The Boss's dying wish. He recruited Ocelot, Big Boss, EVA, Sigint and Para-Medic. This organization was called, The Patriots.
Metal Gear Solid: Peace Walker
Peace Walker Incident
Big Boss, however, came to resent the god-like status that Zero was forcing upon him, and he believed that Zero had misinterpreted The Boss's dream. He split from The Patriots, allying with Kazuhira Miller to form Militaires Sans Frontieres, his own interpretation of The Boss's will modeled after (and funded by) the concept of "Army's Heaven" espoused by Gene in San Hieronymo. Following MSF's counter-infiltration of Costa Rica to stop the Peace Walker Project from culminating in the launch of a live nuclear missile from an AI tank, Zero revealed The Patriots' true hand in the affair when the military company-nation became a nuclear power in its own right.
Using the constant prison escapes of KGB agent Vladimir Zadornov as a diversion, the presumably pacifist Paz Ortega Andrade would sabotage and eventually commandeer MSF's nuclear deterrent, the AI/pilot-driven Metal Gear ZEKE. When Big Boss confronted the hijacked ZEKE atop Mother Base, Paz revealed herself to be the agent "Pacifica Ocean" of "Cipher," and extended Zero's offer to return to the fold of the Patriots and join the winning side as the age of nuclear deterrence gave way to the age of weapons control. Upon Big Boss's refusal, Pacifica resolved to use the nuclear-armed ZEKE to strike the eastern United States, thereby painting MSF as a nuclear-capable extremist military cult, but was thwarted when Big Boss and MSF's support weapons plunged ZEKE into the sea.
Following the battle with ZEKE, Miller revealed to Big Boss that he knew of Zadornov and Pacifica's ties to Cipher/Major Zero all along, trying to exploit them to encourage MSF's growth into the model private military company of the future. Big Boss simply dismissed this admission of subterfuge, instructing the staff of MSF to begin moving its assets, returning to the mobile mercenary base that would eventually set up roots once more as the military state of Outer Heaven.
Despite Big Boss's refusal to rejoin the Patriots, Zero had accomplished numerous goals related to his intended role in the organization - raising a legend of his abilities as a soldier, proving the private military company as a superior control of force to national militaries, and securing Big Boss's "soldier genes" to be expressed in the form of the next generation of super-soldiers and world leaders - the fruits of "Les Enfants Terribles," Solid Snake included. However, Zero's obsession with absolute control over the global state of affairs would drive the other members of The Patriots to leave as well, leaving only Sigint and Para-Medic completely loyal to Zero. Zero became power hungry, and he created the four Patriot AIs, so that they could govern the world when he died.
Metal Gear Solid 4: Guns of the Patriots
At the end of the game, Zero is wheeled into the cemetery where Solid Snake and Big Boss meet for the final time. He is now 105 years old, and exists purely in a vegetative state. Big Boss says that "it all started with zero, and must end with zero". He cuts off Zero's air, thereby killing the founder of the Patriots.
Metal Gear Solid: Portable Ops
Despite presumably being held prisoner for the duration of the game's events, Major Zero may be used as a unique playable character by saving/loading a repeat playthrough with a save for Metal Gear Solid: Digital Graphic Novel on the same Memory Stick, or by inputting the password "1+2-3" (US)/ "GALAXY-0" (EU). If he is not acquired in a Portable Ops save, he is also available in Portable Ops Plus as a reward for finishing Boss Rush mode using imported MPO save data, along with all other missing (unearned) MPO uniques.
Major Zero appears as a card in the MGS3 Pack in both Metal Gear Acid games, acting as an Equip card that emulates the function of Snake's "Active Sonar" radar by briefly showing enemy positions in Search Mode.
- Acid 1 (188 - Major Zero): "Active Sonar" - Displays enemy positions in Search Mode for 1 turn. Provides Red Interference to the north and south. 15 COST
- Acid 2 (176 - Major Zero): "Active Sonar" - Displays enemy positions in Search Mode for 1 turn. 10 COST
- Acid 2 (460 - Major Zero+): "Fox Command" - Displays enemy positions in Search Mode for 3 turns. 4 COST