Sergei Kozin (Russian: Сергей Козин) is a prisoner of the Vorkuta gulag. Nicknamed the "Monster of Magadan," he possesses brute strength, tackling two guards at once and killing them both, killing another guard with a pickaxe uppercut, and body slamming on an armory locker door to crack it open; he also has the hulking build to fit the name; apart from that, little is known of him.

Sergei is seen in the mission "Vorkuta," as part of the band of prisoners who took part in the uprising. He appears on the way to the elevator, pushes the coal cart with Reznov, and disappears after he opens the arms locker. He reappears before the gate to main armory. He uses his strength to hold the door open for Mason, Reznov, and the other prisoners, but just as Mason is able to get through, Sergei is shot and the door comes down on top of Sergei, killing him

No matter how fast the player tries to open the door to save Sergei, he will still die.