Alexsi Ivanovich Smirnov is an agent with the NKVD.
Well, he is for a while. He is not one when we first meet him, understandable since he is just 16 at the time but he will find himself being forced to volunteer to join so he does because Smirnov is above all things a survivor and when it is put to him that he can enlist or get killed, he makes his mind up pretty fast. Which is why I put it that he is with the NKVD.
Now, as time goes along, he will change employers. This will again be because he is not given much of a choice. In this case he is told he has to go to work for the Germans. This being now as the Second World War is starting (the Great Patriotic War to the Soviets). This forced recruitment will come by way of orders from the NKVD and it will land him in the Abwehr, or German Military Intelligence, so I could have written that he is an agent with German Intelligence.
Then there will be the time that the Gestapo decides his rightful place is in their ranks working for them.
And later British Intelligence in the form of MI6 will throw in their two pence.
So who does Smirnov actually work for? Himself. First and foremost if not only.
Our initial contact with Smirnov is as he has teamed up with nomadic raiders living in the area of Iran touching the Soviet Union's republic of Azerbaijan. He is an orphan who fled the Soviet's care to live on his own as a thief and then, with those raiders, a smuggler. His skills with languages (Russian and Farsi, and, surprisingly, German) as well as his lock-picking and other larcenous abilities made him valuable to them and they in turn taught him tricks at stalking, hunting, and survival. How to kill enemies out to hurt him, he already knew but they improved that as well.
What Smirnov did not know was that he was being hunted by the Soviets for reasons other than his criminal activities. They had a special need and their records indicated he was just person for the job.