Hitman

Near an asylum, a group of young bald boys are receiving tattoos of bar codes on the back of their heads; men observe, direct, and coordinate the training of the boys in firearms, hand-to-hand combat, stamina and strength. The boys are trained from an early age to be professional, international hitmen-for-hire.
Years later, Interpol agent Mike Whittier arrives at his house and discovers Agent 47, where the two talk about 47's life as a professional hitman, working for a mysterious unknown paramilitary group known as The Organization.
Three months prior, 47 (Timothy Olyphant) is completing a hit on a gang leader named Bwana Ovie in Niger. He is told by his Organization contact, Diana, to kill his next target, Russian President Mikhail Belicoff publicly. He completes his mission, but before he can leave Russia, he is contacted by his employers. They tell him that there is a witness to the assassination and order him to intercept her. When Agent 47 realizes she has never seen him before, he does not shoot her, instead, he narrowly avoids an assassination attempt on himself, attempted by another unknown Agent.
His employers reveal his location to agents of FSB, who make plans to intercept him. As he is about to be taken, Diana calls to warn him and that Belicoff ordered the hit. After an escape from the hotel, 47 intercepts Nika, Belicoff's mistress, the woman who supposedly witnessed his hit. He interrogates her about Belicoff and discovers that Belicoff had a body double, who ordered the hit on the real Belicoff so that he could take his place as the president of Russia. 47 was to be killed after the hit to ensure total secrecy.
As Nika and 47 attempt to take a train further into Russia, they are intercepted by more assassins from the Organization. 47 kills three before disarming Agent Whittier and wounding his partner. The FSB and Agent Marklov order Interpol to leave the country. 47 contacts Agent Carlton Smith of the CIA. He offers Smith a dealâhe will kill Udre Belicoff, Mikhail's brother and an arms dealer and slave trader, in exchange for a favor. Agent Smith informs 47 that Udre had been planning something with a German arms dealer named Price. Intercepting him could lead 47 to Udre.
47 and Nika travel to Istanbul, where 47 abducts Price in order to pose as Price at a meeting with Udre. 47 kills Udre and his henchmen, so Belicoff's double is forced to attend Udre's funeral. 47 kidnaps Agent Marklov and forces him to have his own agents try to shoot Belicoff's double as he delivers the eulogy. Disguised as a soldier, 47 kills Belicoff's double's guards and takes him into the archbishop's chamber in the church. 47 kills him and allows himself to be taken into custody by Interpol and Agent Whittier.
Agent Smith delivers on his end of the deal, having the agents under his command intercept the Interpol convoy that is transporting 47 to the airport, giving 47 the distraction necessary to escape. The scene then flashes forward to the conversation between Agent Whittier and 47 occurring at Agent Whittier's house. 47 suggests Agent Whittier go along with 47's plan, saying that he will notify the police that he has killed the fake Agent 47, implying that Whittier will not survive if he does not. He also suggests that Whittier waits until after his daughters have gone to school, so they don't have to witness it.
Nika picks up an envelope from an undisclosed sender. Inside it, there are papers and a message saying that she now owns a vineyard. 47 is watching her from afar, through the scope of a sniper rifle. He then looks at the corpse of another Organization hitman, lying close to him, and walks away.

Question: Who is Belicoff's mistress?
Answer: Nika