Stalker

"This is what happens to you if you resist...or if you're just in the wrong place at the wrong time."

- Alyx Vance

Stalkers are humans who have been drastically altered, both physically and mentally, through extreme, brutal Synth engineering by the Combine.

Overview
Barely recognizable as human, Stalkers have undergone drastic surgical procedures and synthetic implantation. A Stalker's limb extremities (hands and lower legs) have been removed and replaced with bolted-on metallic augmentations, which offer reduced agility. Apparently unable to talk, they utter roars and groans, probably as a result of severe larynx and mental alteration.

For ease of maintenance and to ensure a dependence on the Combine, the Stalker's digestive tract has been removed, resulting in them surviving solely on a nutrient solution administered via the gastric port in their sides. Stalker bodies are extremely atrophied, with very little muscle or fat tissue remaining. Their facial features have been replaced by a metal plate, their "eyes" are mere pinholes, and all signs of their genitals have been surgically removed.

They have small plates on hinges next to their "eyes", which can be closed during periods of inactivity. In Half-Life 2: Episode One, during the trip in the stalker train, these plates are closed while the stalkers are in transport. After the train crashed the plates opened, which activated the stalkers.

The Stalkers are naturally unsettling in both appearance and the sounds they make, this is show in Half-Life 2: Episode One when Gordon and Alyx are riding the Razor Train containing them; Alyx is shown to be visibly disturbed by them and when the train crashes, one is shown screaming at Alyx which unsettles her, which is unusual given her often fearless nature.

Application

 * The Stalker transformation process is performed at Nova Prospekt, after which they are transported to the Citadel, where they perform a single job that others nexus don't need, where they are required to maintain the core and guard from intruders.
 * They are transported by Razor Trains, and are confined in pods during the transportation.
 * Despite being seen in Nova Prospekt and the Citadel, Stalkers are never actually engaged in combat during Half-Life 2. However, it is possible to engage in combat with them in order to more easily achieve mission objectives in Half-Life 2: Episode One, as their lasers prevent certain tasks from being completed.
 * Episode One is also where the exact nature of Stalkers is first revealed. In Episode One, the player and Alyx enter the Citadel. After a bit of progress, the player and Alyx enter a room with two Stalkers working a Forcefield, temporarily preventing progress. Alyx states that the Stalkers "shouldn't bother us if we leave them alone". Interestingly enough, when the player is in the core of the Citadel and chooses to kill one of the two Stalkers, the remaining one will turn to face the player and roar at Freeman. The Stalker will then proceed to attack the player with the laser tool. Contrarily, if the player is fast enough to cross a field bridge there before it deactivates, Stalkers can be approached face to face, and do not attack the player but instead walk away as if he wasn't there at all.
 * It appears that the Citizens of City 17 have a full knowledge and understanding about Stalkers, what they are, and how and where they are "made." Alyx Vance comments on Nova Prospekt: "It used to be a high-security prison, it's something... much worse than that now." Barney Calhoun also makes the comment "He was about to board the express to Nova Prospekt!" at the start of the game, and sometimes Resistance members say, "They're not gonna make a Stalker out of me". However, this knowledge may just be limited to members of the Resistance.
 * It could be likely that new arrivals at City 17 were sorted out if they could live as normal citizens or be identified as "unfitting" and be sentenced to Nova Prospekt. At the very beginning of Half-Life 2, this is evidenced when Gordon Freeman arrives at a security terminal and a Combine guard inspects several citizens and orders them to specific locations. Furthermore, as Barney Calhoun mentioned, he barely saved Gordon from being sent to Nova Prospekt on train.

Behind the scenes

 * The Stalker was originally planned to be fought in Half-Life 2, but this feature was cut and, later, re-introduced in Episode One.
 * The Stalkers in the Half-Life 2 Beta had melee attacks, their laser attack did more damage and they would emit a high pitched scream when killed.

Trivia

 * Stalkers can be spawned via the console in Half-Life 2, but will the game will crash when killed, due to lack of a ragdoll model, this has been fixed after a update. When spawned, they will simply walk around and occasionally shoot lasers. In Episode One, spawning works normally. In Portal, they are spawnable but stretch when killed, similar to Half-Life 2.
 * Stalkers spawned with the console in Episode One are generally non-aggressive, and will usually not fight if other Stalkers are killed in front of them.
 * There is only one Stalker-related achievement, Pacifist, which can be gained by containing the Citadel Core without killing any Stalkers.
 * SMOD has a full working Stalker NPC that attacks the player and other non-Combine NPCs.
 * A spawned Stalker and a spawned Vortigaunt will automatically fight, suggesting that the two were originally meant to.

List of appearances

 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Episode One