Stalkers are humans who have been drastically altered, both physically and mentally, through extreme, brutal Synth engineering by the Combine. They are servants, mindlessly operating machinery and guarding the core in the Citadel.
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 and result in a slow, zombie-like gait. Apparently unable to talk, they utter only 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 Stalkers' digestive tracts have been pulled out, resulting in them surviving solely on a nutrient solution administered via the gastric port in their sides. Stalker bodies are extremely atrophied and skeletal, 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 Their facial features have been replaced by a metal plate, their "eyes" are mere pinholes, and all signs of their genitals have been surgically removed.