Seth

Seth was a former Militaires Sans Frontières member and a mercenary, as well as a war buddy and friend of the Captain. However, during the disastrous Ground Zeroes Incident, he ended up sucked into a wormhole to Dite despite the Captain's attempts to save him.

At some point later, he was found by the Charon Corps, and initially attempted to aid them. However, he ultimately allowed himself to be infected by the Wanderers' Death Dust and proceeded to kill the unit save for Dan. He was nearly tortured by Dan as revenge for what happened to the Corps, although the Captain's arrival forced him to retreat. Seth then blamed Dan for the attack and implied that the latter was psychotic.

After recovering Sahelanthropus, he revealed his true colors and proceeded to abduct Chris and forced Virgil AT-9 to accompany him to the Base Camp. He then let slip that he was in fact a Wanderer himself, and also proceeded to reactivate the Death Dust inside the AI, and also attempted to convince the Captain to succumb to his Death Dust infection. However, Reeve shot him, with Seth fully transforming into a Mortar-type Wanderer, forcing the Captain to put him out of his misery. Despite his death, he ultimately succeeded in infecting the Captain and Virgil with the Death Dust.

Personality and traits
He was implied to be one of the higher ranking or at least one of the more experienced MSF members due to his reunion with the Captain having him mention that the latter was "greener than grass".

Behind the scenes
Seth is a major character in Metal Gear Survive, as well one of only two actual boss characters in the game (the other being the Lord of the Dust itself).