According to unused leftover dialogs from the game , he is one of Major Khaletskiy's right-hand men (the other being General, the only one who actually made it to the final version). The player would have been able to help Khaletskiy against the Loners by assisting Sokolov's squad, but this have been removed from the final version.
↑st_dialogs_escape, string id esc_comm_talk_11111122: "Daaamn... I bet my left nut it was Sidorovich! I knew he'd eventually try pulling deals behind my back. And he sure picked a good time to stab me in the back!... Oh, well, life's never been fair and never will. Listen, two army squads are ready to attack, one at the depot and another at the mill. They are under the command of Sokolov from the depot. Go to him and let him know I'm alive and well because stalkers don't have the guts to kill me. Tell him to begin the assault."