Born 1978 in Yugoslavia, he worked as a soldier in the Yugoslav Wars during adolescence; as a young man he worked as a smuggler across the Adriatic Sea.
Niko grew up in the same village as Roman, with their mothers and their fathers (who were brothers).Niko and Roman's fathers were abusive alcoholics, whom they both despised; Niko's mother, Milica Bellic, was a long-suffering woman, unhappy that her son grew up in such a harsh place.In a conversation with Patrick Mc Reary, Niko reveals that he had a brother who died during the Bosnian war.According to the dialogue in the mission, Taking in the Trash, Niko dreamt of becoming an astronaut when he was a child.In a conversation with Dwayne Forge, Niko says in the place he grew up, they did not have electricity until he was twelve.
Niko grew up in the harsh environment of the Yugoslav Wars, and became a soldier himself as a teenager (The Yugoslav Wars being between 19, this means Niko was only 13-17 years old at the time).
Niko says that he was betrayed "ten years ago", making it closer to 1997 or 1998 (although Florian claims that its was "twelve or thirteen years ago" in a phone call after the mission That Special Someone).
He served as a helicopter pilot and an infantryman during the war, and during the mission Easy Fare, Roman alludes to him driving a tank (although given Roman's lack of first-hand experience, this is not necessarily true).
During the war, Niko both witnessed and committed numerous atrocities, which led to his cynical perspective on life, and a certain degree of regret, depression, and emotional detachment.
Eventually, Niko's unit of fifteen men (most of them friends from his village) were ambushed by enemy forces, and all but three were killed: Darko Brevic, Florian Cravic, and Niko himself.
Niko concluded that either Florian or Darko had sold the group out, and he vowed to uncover and kill the culprit, not only to gain revenge, but also to hopefully 'close the book' on the incident, and move on with his life. In 1998 Niko's cousin, Roman had already moved to America to start a new life in Liberty City.