An Up to Eleven version of the Tyrant tasked with killing the remaining members of S.T.A.R.S. from the Mansion Incident, appearing in 3 as The Heavy. With the exception of Combat Tentacles, and the BFG, and the ability to talk, "Mr X" from 2 is identical. Which is only natural, seeing as how Nemesis is an upgraded version of Mr. X with a dose of extra Body Horror.
Badass: Years after his only starring role, fans of the series still herald Nemesis as the bringer of their childhood nightmares, cite him often as the second best villain in the series. The game is named after him for a reason; this is not the Tyrant, this is not poor ol' Dr. Birkin, this is not Wesker, this is The Nemesis. And you will remember his name.
Supplementary material including one of the Wesker Reports and the Archive books explain that he's one of the factory-line Tyrants like Mr. X, plus additional abilities granted by the parasite implanted in his body. Further material explains that Wesker and Birkin obtained the same parasite to test on poor Lisa Trevor in addition to the other viruses and diseases she was subjected to, causing her own Implacable Man status during the Mansion Incident.
Healing Factor: Thanks to that good ol' parasite boosting his brainpower, Nemesis can actually heal himself via mass consumption. And if his chosen meal is a Tyrant corpse, he'll be jettisoned into a spiraling mutation.
The Heavy: Of RE3, pursuing Jill from the first chapter to the last.
Implacable Man: Throughout the game, at any time, anywhere, Nemesis can attack you. You can drive him off temporarily if you have the ammo, but he'll attack again sometime soon.
Lightning Bruiser: Unlike Mr. X, Nemesis can sprint damn fast and does so often when pursuing Jill.
Made of Iron: The Nemesis is nearly unkillable from conventional weapons...
Neck Lift: Prior to getting his coat burned off, he uses this as a finisher.
No Plans, No Prototype, No Backup: Another unexplained thing about Nemmy is why Umbrella never made more than one of him even though he's stronger, faster, smarter and significantly more resilient than the Tyrants they did mass-produce.
Supplementary materials suggests they did; one seemed to be resisting control and had to be destroyed, the other two are unaccounted for.
Puppeteer Parasite: Controlled by one, which accounts for his increased intelligence over base model Tyrants.
Recurring Boss: Notably, while there are several scripted fights, Nemesis can attack you at any point in the game.
Urban Legend of Zelda: There use to be a persistant fan rumor that Nemesis was secretly one of the series' pre-existing characters who was captured and mutated by Umbrella (this was mostly due to a mis-translated interview with one of the Resident Evil developers that wrongly implied he was supposed to have been a significant character before). They ultimately ended up using this angle with his character in the movies.
Voiced By: Roger Honeywell (1999) and Rick D Wasserman (2012)
A sergeant for Umbrella's U.B.C.S. in 3 also secretly tasked with gathering data on the numerous B.O.W.s roaming the city - of course, for a hefty price, which brings him to the decision to kill all others on the operation.
The Ahnold: he talks this way in Resident Evil Outbreak. His accent in RE3 and Operation Raccoon City is more "Russian" in character.
Ax-Crazy: He seems pretty grounded and as rational as you can say a sociopath trying to assassinate his own team is in the games, but in the sections of the S.D. Perry novelizations from his point of view, he really likes what he does.
In RE3 and Outbreak, Nicholai is calm and dispassionate, even while he's setting up the rest of his team to get killed. In Operation Raccoon City, he spends two levels trying to take out the Wolfpack, and comes off like he's one facial wound away from going full-blown monologue-spouting Bond villain, complete with rigging up half the zombies in the hospital with suicide vests.
Badass Normal: Somewhat unusual for a Resident Evil villain, he never goes One-Winged Angel. In fact, he's probably one hundred percent baseline human, yet he survives against ridiculous odds two or three times.
For the Evulz: In Operation Raccoon City he feeds people to B.O.W.s solely to see what happens when you feed people to B.O.W.s (although this is presumably his interpretation of "gathering combat data").
Mad Bomber: In 3 he blows up a hospital. In Outbreak, he blows up a university. And in Operation Raccoon City, he goes so far as to rig dozens of zombies to explode upon attacking the Wolfpack.
Made of Iron: He swallows a lot of punishment for a guy with no viruses in him. Depending on player actions, it's possible for him to somehow survive being inside a gas station when it explodes (in fact, he was practically leaning over the epicenter of the blast), and then get blasted out a forth-story window, with no apparent ill effects.
Like every other boss character, he gets a double dose of this in Operation Raccoon City. You can put about two dozen .30-06 rounds through him at one point, doing enough damage to repaint the wall behind him, and it doesn't kill him. In fact, the fight ends when he's survived for long enough that your handler loses patience and makes him a lower priority.
Murder Is the Best Solution: He wants more money from Umbrella, so his solution is to kill all the other supervisors; therefore giving him a monopoly on the information.
Spell My Name with an S: His name should be "Nicholai Zinoviev" (Николай Зиновьев), a result of his name being translated from Russian to Japanese to English.
Sociopathic Soldier: In spades. The U.B.C.S. and U.S.S. have their fair share of these, but Nicholai is the standout.
Unfriendly Fire: Apparently spent most of his time in Raccoon tracking down and killing other Umbrella monitors to have a monopoly on Bioweapon combat data.
Villains Never Lie: Interestingly, Nicholai does not lie to Jill at any point in time. He attacks her in a couple of possible scenarios in RE3, but he is always absolutely truthful.
What Happened to the Mouse?: In one of the three endings, he anticlimactically escapes Raccoon City in a helicopter, leaving him a loose end. He dies in the other two. We have his reports to Sergei Vladimir, so cannonically he's alive, but he hasn't been seen since.
Voiced By: Ben Campbell (1999)
The leader of the UBCS platoon that Carlos and Nicholai belongs to. He is already mortally wounded by the time Jill meets him.