Factions
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Duty
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Duty is a paramilitary clan of stalkers operating in the Zone with members living according to a code. They, along with Freedom, are also one of the most powerful stalker clans in the Zone.
The founding members of Duty, as well as many of its subsequent members, were originally Ukrainian Military troops and Special Forces units sent into the Zone and left to die there after being nearly killed by the Zone's mutants and anomalies. Other members are ordinary stalkers who have seen enough of the Zone's horrors, and have decided to do something about it. Duty sees the Zone as an ulcer in the world that must be purged at all costs, as they believe that if left unchecked, mutants will soon break out of the Zone and attack the outside world. They achieve this by killing all of the Zone's manifestations, namely Mutants, in the process bringing them into conflict with factions such as Freedom, who believe the Zone is something beneficial. Duty also fights against other factions such as the Bandits and Mercenaries, who extort Stalkers for personal gain. With superior equipment and fervently dedicated members, they fully intend to achieve their goal.
Duty is at odds with everyone seeking to exploit the Zone and, as Duty sees it, spread its corruption. They opt for the complete destruction of the Zone through brute force, effectively attempting to "kill" the Zone. They are contrasted by other factions, such as Freedom or the Ecologists, who see the Zone as a miracle. They tolerate Loners, but are hostile towards Freedom faction members, seeking to undermine their efforts at every step. They also openly engage Bandits due to their hostility and extortion of free Stalkers.
They are, however, openly cooperating with the Ecologists at Yantar, transferring all recovered artifacts to them; so that the Zone can be better understood (and, as Duty wishes, defeated).
The faction has long been in fierce conflict with Freedom due to their conflicting ideals.
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Freedom
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Freedom is a clan of stalkers who fight for free access to the Zone, and their views are somewhat anarchistic, believing nobody can own the Zone and seeing it both as a scientific marvel and as a free life inside the Zone's borders. Because of their ideals they constantly find themselves in conflict with the Military and their long time nemesis, Duty. They view Duty as no more than a clan of jarheads who see nothing, hear nothing, and only know how to pull a trigger, as instead of finding a way to co-exist with the Zone, they attempt to destroy it, whereby the Zone retaliates with mutant attacks, anomalies, radiation hazards and blowouts. Similar in size and manpower to Duty, they have the offensive strength but medium and at times low resources to wage a faction war against Duty.
In contrast to the regimented, highly disciplined atmosphere of Duty, the members of Freedom have a much more relaxed, informal attitude. They appear to have a more casual command structure, often referring to each other as "bro", with a number of members shown making regular recreational use of alcohol and marijuana. This further reflects their status as a clan of like-minded free spirits, unlike the ex-military soldiers of Duty. Regardless, Freedom members are able to function very effectively as an organized fighting force when necessary.
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Mercenaries
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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, or possibly 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.
Mercenaries often charge their clients large amounts of money or anything of the same value (artifacts for example). They are usually hired by wealthy and powerful corporations to retrieve artifacts and other unique objects in the Zone.
Because mercenaries have no established code of ethics to abide to, each mercenary group's sense of morality could be quite different from another. Thus, mercenary groups are ultimately unpredictable as there are those who are civilized enough to negotiate and reach a mutual understanding, such as Hog and Hatchet, while there are also those who will simply kill anyone that gets in their way or witness their brutality, such as Wolfhound.
It is this bit of unpredictability than has earned all mercenaries the distrust of most stalkers as they fear that they might get a bullet in the head than a mutual understanding when talking to mercenaries.
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Bandits
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Bandits are a group of mostly ex-criminals who came in the Zone either to escape from the law, trade weapons or make money. Bandits are ruthless and generally hostile to anyone not in their gang. Though depicted in S.T.A.L.K.E.R.: Clear Sky as a united, highly territorial faction, the Bandit population in S.T.A.L.K.E.R.: Shadow of Chernobyl and S.T.A.L.K.E.R.: Call of Pripyat consists mainly of roving groups. In Call of Pripyat, most of the bandits are under the leadership of Sultan, although in Yanov most are led by Jack and a small group is led by Shishak.
They are one of the most poorly-equipped factions of the Zone, usually relying on low-grade weaponry and light armors. However, in Clear Sky, in Limansk, one can encounter well-armed Bandits equipped with SA Avalanche assault rifles, led by an Exoskeleton-clad expert wielding an RP-74.
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Loners
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Loners represent the stalkers who haven't joined any faction and operate in the Zone by themselves. Even though they're described as not a proper faction of their own as per se, it is shown in all three S.T.A.L.K.E.R. games that the Loners seem to act as a faction, as seen by the fact all Loners are friendly to each others, and that being friendly/hostile to a group of Loners results in hostility to all Loners. They're also all seen using the same gear. However, unlike most factions, they do not have a single leader - much like Mercenaries, Loners include several groups led by chosen leaders, such as the Rookie Village group led by Wolf and later Fanatic, and the Zaton loners seem to be under the leadership of Beard.
Their main rival faction are Bandits, who have a history of preying on stalkers to extort them. They are also hostile to the Monolith although there are few encounters between them.
The loners also hold a shaky relationship with the Military. The loners are involved with the military in the illicit trade of firearms and artifacts in the Zone, not to mention numerous intelligence reports about the Zone itself and its happenings (where the expenditure of professional soldiers is deemed too risky). However, the military do have the power to shoot stalkers on sight and have been known to abuse their power over stalkers, which generally makes most Loners resentful against the military. In Call of Pripyat, they are neutral to the army, although the only stalker to have been in contact with the army was Garry, who managed to be allowed by Colonel Kovalsky in their base in exchange of food.
They also hold an unfriendly relationship with most mercenaries within the Zone; this is most likely attributed to the mercenaries' reputation of being secretive and somewhat unpredictable individuals who will not hesitate to kill anyone who gets in the way of their work. While they won't normally shoot mercs on contact, they will commonly deny them access to their territories. In Call of Pripyat, this is no longer the case, and groups of mercs and loners can be seen gathering at the School and the Yubileiny Service Center, defending their camp against mutants and occasionally speaking to each other.
They are neutral to the Freedom, Duty and Ecologist factions.
Additionally, while the game calls them "loners", people in the Zone never actually refer to them as such. Loners are usually addressed as "stalkers" or "free stalkers" by the Zone's inhabitants.
