Venom Snake

Punished "Venom" Snake, also known as Ahab or Big Boss, was a medic who worked for the military unit Militaires Sans Frontières in 1975 and a military commander in 1984 and 1995. In 1984, facial surgeries permitted him to act as a double for Big Boss, after the former medic passed 9 years in a coma, and further lead the military unit founded by his former boss as a successor to MSF, Diamond Dogs. While commanding the unit, Venom Snake defeated Skull Face and his XOF unit.

Punished "Venom" Snake became the phantom of Big Boss and his traits and further actions while leading the Diamond Dogs departed from the former idea of Big Boss as a hero, becoming the phantom of Big Boss. In 1995, Venom Snake, now known as Big Boss, commanded the military fortress Outer Heaven. He was finally killed by one of Big Boss clones, Solid Snake, in an operation commanded by Big Boss himself.

XOF Trojan Horse operation
In March 1975, the medic was stationed on board the helicopter sent to extract MSF commander Big Boss and rescued POWs Chico and Paz Ortega Andrade.

During the flight back to Mother Base, Big Boss ordered the medic to look over the unconscious Paz, after Chico noticed surgical scars on her abdomen. Suspecting that a bomb had been concealed inside her, the medic was forced to operate on her immediately, without time to administer anesthesia. During several intense moments, the medic successfully removed the bomb from her gut, as Big Boss and Chico held her down.

Although the bomb was quickly disposed of, their efforts were for naught, as a second bomb had been planted inside her. After coming to, Paz, aware of the impending detonation, jumped from the chopper to avoid the MSF personnel being killed along with her. As the medic shielded Big Boss, the bomb detonated and his body bore the brunt of the explosion, somewhat shielding Big Boss, before the helicopter was sent into a collision with an enemy chopper.

Behind the scenes
Punished "Venom" Snake is the main playable character of Metal Gear Solid V: The Phantom Pain.

The unidentified field medic appears during the closing cutscenes of Metal Gear Solid V: Ground Zeroes. Because of the cinematic angles during the ending scenes in Ground Zeroes, there is never a clear view of the medic's face. However, the medic's full appearance has been seen in the PC version through the use of mods. A PC user extracted the facial model and textures for the medic and found that he shared identical scars to Big Boss.

Fulfilling his role as the phantom of Big Boss, he returns as the main antagonist of the original Metal Gear via a retcon established in the ending timeline.

On the Air Command Center menu, if the player look closely at Venom Snake's reflection, it shows the player avatar the player created, hinting at the revelation later of how Venom Snake isn't actually Big Boss. In addition, a leaked Italian version of the Piggyback game guide for Metal Gear Solid V: The Phantom Pain strongly implied Venom Snake's true identity is separate from Big Boss, although they never explicitly confirmed it due to Konami and at that time Kojima Productions specifically forbade them from giving any statements about Ishmael's identity.

It is heavily implied that Venom Snake himself (and his nondescript identity as a medic) serves as a player avatar, due to his original appearance being reliant on the player's decisions and how Big Boss addresses him in a manner that breaks the 4th wall.