*Massive spoilers for Metal Gear Solid V ahead!
The man who would eventually become known as Venom Snake was one of Big Boss’s medical staff as part of Militaries San Frontieras mercenary unit. According to Big Boss, he was one of his best men. During the mission to a Cuban prison to rescue Chico and Paz, he was part of the helicopter crew that extracted the two, along with Big Boss.
However, it turned out Paz had been booby-trapped-and MSF had wondered into a trap, with a nuclear inspection turning out to be an attack by the rogue Cipher group XOF. Although he was able to remove one device from Paz, the medic was unable to locate the second in time, and Paz threw herself out the helicopter door. The resulting explosion killed not only Paz, but also Chicho and the helicopter pilot. Kaz is able to survive with relatively minor injuries. The medic put himself between the explosion and Big Boss-and the two lapsed into a coma.
In the next nine years, the two were moved to a Cyprus hospital by Zero, who although he and Big Boss were enemies, he still felt the world needed Big Boss (and the attack on Mother Base was not his doing, but that of his XO “Skull Face”). Zero-along with help from Eva and Ocelot-begin to modify the medic into a decoy, using plastic surgery and hypnotherapy despite the coma, making the medic think he was Big Boss. Despite this, there were still differences-Big Boss’s body was relatively intact, while unfortunately, his decoy was missing an arm and had shrapnel in his head, as well as more visible scars. He also, due to the shrapnel, has a tendency to hallucinate, and while he believes he is Big Boss and has knowledge of his missions, his personality is somewhat more withdrawn and quieter than the more talkative real Big Boss.
However, Big Boss recovered earlier, and talked over the plan with Ocelot. He then took up the identity of “Ishmael”-wrapping his head in bandages to continue the deception. When the medic woke up, he’s helped by Ishmael-who dubs him “Ahab”-to escape the hospital from an XOF attack-which includes the mercenary later known as Quiet, as well as The Man on Fire-A resurrected Colonel Volgin-and the boy who would one day be called “Psycho Mantis”.
Eventually escaping their pursuers, “Ishmael” vanishes, and the medic-dubbed “Ahab”-is recovered by Ocelot. Ocelot helps “Big Boss” retrain his atrophied muscles, gives him a cyborg arm prosthetic, and helps him rescue Miller from Afghanistan.
The trio-with the medic now more or less going by the name “Punished Venom Snake”- then form Diamond Dogs, designed in part to get back at Cipher/XOF, creating a new Mother Base. After rescuing Huey Emmerich, who Miller believes betrayed Mother Base and now works at XOF, they uncover a plot by Skull Face to build a new Metal Gear and spread a virus which will infect anyone speaking English, a twisted version of the Boss’s vision to unite the world.
During his missions-which cover Afghanistan and Africa-Venom Snake recruits new allies, including Quiet, the woman who tried to kill him at the hospital; the two sort of have a complex love/hate relationship, although Quiet is unable to speak for fear she will activate the virus (which she is a carrier of).
With Ocelot’s help,He also raises a Wolf pup into the scout D-Dog.
He also encounters the young “Liquid Snake”, Eli, who believes Venom Snake is his father Big Boss. After Skull Face is defeated, unfortunately there’s still problems. Emmerich unethically tries to experiment on the parasites, causing them to mutate and infect several mother base soldiers on the Quarantine platform. Snake is forced to kill several of his own men to contain the virus. Cremating them, he decides to make their ashes into diamonds, making them true “diamond dogs”. He also exiles Huey, spearing him an execution.
After this, Quiet leaves to try to save Mother Base from another infection, but is captured. Rescued by Venom Snake, the two face off against a large force of tanks and helicopters, but both are wounded and caught in the sandstorm-and Snake suffers a literal snake bite. Quiet is able to call up a helicopter and save Venom, but then vanishes for good.
Ultimately, the medic’s old memories begin to surface, and he is briefed by the real Big Boss, currently in hiding.
“Now do you remember who you are, what you were meant to do? I cheated death thanks to you. And thanks to you I’ve left my mark. You have too. You’ve written your own history. You’re your own man. I’m Big Boss, and you are too. No, he’s the two of us together. Where we are today, we built it. This story, this ‘legend’, it’s ours. We can change the world, and with it, the future. I am you and you are me. Carry that with you wherever you go. Thank you, my friend. From here on out, you’re Big Boss.”
Venom continues to be the proxy Big Boss while the real Big Boss forms FOXHOUND, training Solid Snake. The two work together to build the Outer Heaven nation and compound, and it’s Venom who encounters Solid Snake at the end of the original Metal Gear, where he is killed in combat by Solid Snake.
Venom Snake’s sacrifice however allows the real Big Boss to go into hiding, and emerge later on with an even more powerful state-Zanzibar Land.