Kozlow was the one who murdered young Nathan Price's biological mother. As a way to get information out of her husband Martin. Frank Burton, who knew Martin wouldn't be able to raise Nathan, he put the boy in foster care.
Kozlow then sent his henchman to put the picture of a young Nathan on a missing kids website. And years later his trap worked. And Kozlow went to get back what Martin stole from him a while ago.
Kozlow and his right-hand Sweater try to find Nathan and his crush Karen Murphy in a hospital. But Nathan's psychiatrist Dr. Bennet is able to get them away from Kozlow.
Later on during the battle on the train, Kozlow's right-hand man Sweater is killed by Nathan. And Burton is able to extract Nathan and Karen from their route and bring them to a restaurant.
Here Burton reveals Kozlow is after an encrypted message of people who have sold state secrets. A list Burton is on. Kozlow and his henchman attack them, but Nathan and Karen manage to escape unharmed.
Using Sweater's phone Kozlow contacts Nathan who arranges to meet Kozlow at a Pirates baseball game, and if they don't show up he will kill Karen's parents. Using a conducted plan Nathan and Kozlow meet in person for the first real time. Here Kozlow reveals he killed Nathan's mother and several other secrets, Kozlow is also able to intercept Nathan's plan of shooting him. A chase ensues thereafter around the stadium, until Nathan is told by his real father to lead Kozlow outside.
As Nathan with an injured leg flees the stadium, Kozlow has already caught up with him and threatens to shoot him, mocking for saying he is no better then his father. Nathan replies "you sure about that" to his mortal enemy. And then Kozlow's gaze focuses on a blink from a nearby building, realizing who it is he yells one last time "Martin..." before Nathan's dad takes a shot and shoots Kozlow down, killing him.