Stalker is an antagonist of the superhero comic known as Extinctioners, he is one of several humans known as the Hunters who were sent to Alden in the past to ready the planet for the arrival of humans and recolonization at the word of Noah Adam Mahn, however due to an accident they lost contact with the rest of humanity and as a result began to utilize tactics that could be considered war-crimes by today's standards (though it still unknown whether or not Mahn's tactics are criminal or accepted by the other humans, who have yet to arrive on Alden).
As for Stalker himself, his real name is Ethan Stalworth and he wasn't hired to be part of the team for any type of scientific background. Pure and simply he is a hunter. His is a trained field tracker and killer and he enjoys his job, a lot.
While many high tech weapons are at his finger tips, he prefers using low-tech hunting tools to capture his prey. Particularly knives and long range rifles. Many of his fellow Hunters believe he is crazy, a reputations well earned, and it's no wonder he was partnered up with the humanimal Tracker, Lynx Wylde, because both have personalities that compliment one another.
Stalker is infamous for his inclusion in a story with Lynx Wylde in which he raped an erotic Alden dancer and cut off her limbs - apparently this kind of "sport" is very common for the pair and Stalker has no problem with sexually engaging non-human species, probably due to his sociopathic nature in which other life forms are simply "toys".
Stalker is now mutated to survive Alden, since humans can not naturally survive on the planet, the side-effect of said mutation is that he has an anthropomorphic guise - meaning the psychopath can more easily blend in with the inhabitants of the planet he plans on ravaging.