Within the game's storyline, Niko is an ex-soldier from Eastern Europe, who was shaped by his experiences in an unidentified war he fought in, developing a very cynical view on life.

After becoming involved with a Russian crime syndicate, and discovering that his unit was sold out to enemy forces, he decides to move to Liberty City to pursue the American Dream, inspired by his cousin Roman's personal tales of luxury and riches that he had experienced while living there for the past decade.

As the game's story progresses, Niko works for various prominent fictional crime figures, in the hopes of finding the traitor who betrayed his unit during the war, while slowly learning to let go of his past and quest for revenge, and attempting to leave the criminal life that comes with major risks.

When deciding on Niko's background, the writers felt that being an immigrant could lead to more dangerous situations, and therefore more enjoyable missions; after discussions with criminal experts, Houser found that "the real scary characters are not born in America anymore".

Milica, a maternal and caring presence in Niko's life, regrets that her sons have been forced to endure the hardships they did as children, including their abusive, alcoholic father.

As a teenager, Niko participated in an unnamed war as an infantryman, tank driver, and helicopter pilot, during which he witnessed numerous atrocities that traumatised him, leading to his cynical view on life.

Niko later joined the merchant navy to flee from Bulgarin, befriended the crew of the Platypus cargo ship, and contemplated his cousin Roman's requests for him to come to Liberty City in the United States.

In reality, his cousin lives in a small, decrepit apartment in the Broker district,[30] and owes gambling debts across the city, which he struggles to pay through his unprofitable taxi service.

[35][36] During this time, he also begins making criminal contacts that will eventually become important allies,[37] such as Yardie second-in-command Little Jacob and car entrepreneur Brucie Kibbutz.

In the process, he befriends Irish gangster Patrick McReary, and discovers that his girlfriend Michelle is an undercover government agent actually named Karen, who entraps Niko into working for her agency, known only by its cover: the United Liberty Paper (ULP).

After assisting Ray Boccino, a caporegime in the Pegorino Crime Family with an important diamond deal, Niko finally locates Florian, now the flamboyant homosexual Bernie Crane, only to discover he is not the traitor.

He then sets off to find Darko, now revealed as the one who betrayed their unit, while continuing to work for the Italian Mafia in Liberty City, including the Pegorino and Gambetti families.

Niko and Roman eventually secure the lifestyle they sought after the latter rebuilds his taxi company and buys a new apartment in Algonquin using insurance money from his formerly destroyed business.

Having achieved closure on his past, Niko is summoned by Don Jimmy Pegorino for one final favour: to help with an extremely lucrative deal on heroin in collusion with Dimitri.

[39] The former scenario sees Niko successfully killing Dimitri and his men,[40] only for Pegorino, enraged at losing out on a substantial profit, to try and exact revenge on him at Roman and Mallorie's wedding.

Niko is mentioned several times in Grand Theft Auto V (2013), indirectly by Lester Crest, who, when planning a jewellery store robbery alongside protagonist Michael De Santa, considers "an Eastern European guy making moves in Liberty City" as a possible accomplice, before dismissing the idea, claiming that he "went quiet".

Jon Hicks of Official Xbox Magazine and Andy Robinson of Computer and Video Games both called Niko "charismatic" and "likeable", stating that they prefer him over previous protagonists of the series.

"[54] Tom Bramwell of Eurogamer opined Niko "himself is quickly sympathetic - his moral latitude is rooted in horrible war stories, but he's warm-hearted - and imposing".

[57] Yahtzee Croshaw of Zero Punctuation considered Niko an improvement over previous series protagonists, regarding him as "a very human, very relatable character who could still believably lose his mind.