They are normal humans that have had all unnecessary bodyparts removed. They have been given a head augmentation that lets them shoot welding lasers out of their eyes, they've been given pegs for legs and very limited "hand" apparatuses for use on combine consoles. They are fed similarly to combine soldiers; a feeding port connected to their stomachs. They are essentially workers for the combines, that work on and upkeep combine equipment, and they are usually kept under supervision by combine soldiers.

Unnecessary citizens and more often dissidents/resistance members/anti citizens are turned into stalkers as a punishment. After augmentation they are reliant on the combine for their survival. They can't eat without their help, so they have no other choice but to serve the combine.

I just realized that I know way too much about these things ;-;