"I am one of the supervisors, that's all you need to know!"
— Nikolai to Carlos Oliveira, about to kill him.
Nikolai Zinoviev (Russian: Николай Зиновьев), codenamed "Silver Wolf", was a ruthless UBCS operative and Monitor agent in the Umbrella Corporation. He and the USS commando, HUNK were considered to be "rivals".
Originally from Moscow, Russian SFSR, Nikolai served the Soviet Union in one of their 'Spetsnaz' (special forces) groups. Following the collapse of the Soviet Union, Nikolai and a number of other Soviet soldiers were hired by Umbrella to serve in their paramilitary forces.
"You can either accept it with dignity or die with regret! It's entirely up to you!"
Nikolai served as a squad leader -with the rank of Sergeant- when the UBCS was sent into Raccoon City by Umbrella in an effort to contain the T-Virus outbreak. The company, save for a few monitors planted within, was given false orders intentionally by Umbrella in the guise of protecting the city's population. His true nature was as a member of the company's secret internal espionage organization known as the "Monitors", with the covert mission to gather combat data on Zombies and B.O.W.s, along with any other dangers in the infested city. With each collection of information, the agent in question would receive a large monetary reward.
Delta Platoon - which he was assigned to - suffered heavy casualties, with only he, Corporal Carlos Oliveira and Captain Mikhail Victor surviving; Upon the inclusion of former S.T.A.R.S. member Jill Valentine into the group, he knew that the Nemesis-T Type was after her and decided to fake his death in order to not interfere with its mission. They prepared for evacuation to the Saint Michael Clock Tower. Before the tram could be started however, Nikolai was either attacked by zombies or trapped inside the gas station (his fate may change depending on where the player finds Carlos).
Two days later, in the early hours of October 1, he was in the Raccoon General Hospital where he was seen by Carlos, either attacking a fellow UBCS member named Tyrell Patrick, or with Tyrell telling him that Nikolai shot him. Carlos later has to run away from C4 charges planted by Nikolai which later blew up the building. Nikolai went to the Raccoon Park where he met Jill again, revealing his real plans, before escaping upon the arrival of a giant worm.
Nikolai used his position to assume command of a UBCS team tasked with "Operation: Emperor's Mushroom"; to retrieve a sample of blood from a Tyrant named Thanatos. Thanatos proved to be too much for the team to handle, so Nikolai was forced to enact the contingency plan; he killed Greg Mueller with a silenced pistol and then destroyed Raccoon University. This failed, resulting in Thanatos mutating into its super Tyrant form. However, he had escaped long before the building's and the Tyrant's destruction.
Jill met him for a final time in the "Dead Factory", where he commandeered a helicopter - the same helicopter Carlos told Jill about.
The player's choice affects Nikolai's fate.
If Jill jumps off the bridge (canon), Nikolai takes the last helicopter, and Jill has the choice to either blow up his helicopter and kill him, or try reason with him, which allows him to escape.
If Jill didn't jump off the bridge, then Nikolai is killed by Nemesis.
The BradyGames strategy guide for Resident Evil CODE:Veronica features a report written by Nikolai to Umbrella, dated October 13, 1998, he detailed some of the data he gathered there during his mission, which included information regarding the Mansion Incident; the Special Tactics And Rescue Service's investigation of the Arklay Laboratory, and the subsequent destruction of the mansion facade. He correctly speculated on Albert Wesker faking his death to betray Umbrella. He informed Umbrella of the surviving S.T.A.R.S. members and his encounter with Jill, warning them that they were a grave threat to the corporation and should be eliminated. The report is written by Dan Birlew and is not canon.
A timeline in Resident Evil 5 confirmed that Nikolai took the last helicopter (which in turn also confirms that Jill did indeed jump off the bridge) but did not confirm whether Jill shot down the helicopter or tried to reason with Nikolai, officially leaving Nikolai's fate up to the player. The BradyGames-translated reference guide, Resident Evil Archives refers to Nikolai as having left Jill to die by taking the helicopter, but makes special mention of the fact that events described in the book may not be how the player experiences them due to the game's open-ended and multi-choice nature.
↑In the original Japanese version, the final report is signed by the development staff with earlier entries being attributed to Nikolai. In the English-language edition however, all entries are mistakenly attributed to him.