We are the Mercenaries.

The Mercenary faction is a group of private military contractors operating within the Zone, though their exact base of operations is kept highly secret. They appear in S.T.A.L.K.E.R.: Shadow of Chernobyl, S.T.A.L.K.E.R.: Clear Sky and S.T.A.L.K.E.R.: Call of Pripyat.

Mercenaries are experienced fighters from all walks of life who offer their services as hired soldiers. They are perhaps ex-military soldiers, ex-special forces operators, underground fighters such as elite resistance/guerrilla troops, a private military company or notorious terrorists of various nations who offer their services to the highest bidder. Having seen combat outside of the Zone, the mercs are capable of dealing with virtually any kind of armed resistance using careful tactics, military precision, and excellent equipment. However, their only drawback is that they lack the extensive knowledge of the Zone that many Stalkers have.

S.T.A.L.K.E.R Wikipedia on Mercenaries[stalker.wikia.com]