Nero is the don (boss) of the Omerta family. Unlike his right hand man Big Sal, he is mostly absent in the presence of the general public, instead coordinating the shady operations of the casino from the privacy and comfort of his own hotel room. He thus leaves most of the day-to-day business to Big Sal and the lieutenant Cachino. He does not appreciate callers violating his inner sanctum.
Though he pretends to be loyal to Mr. House and the other Strip Families, he is secretly planning to help Caesar's Legion take over New Vegas. In exchange for being given control of the Strip after the Legion's planned conquest of the Mojave, Nero and the Omertas pledged to Caesar to raid the NCR Embassy and the other Families' casinos. Once the Legion has marched across Hoover Dam, they'll be able to take New Vegas without facing resistance from Mr. House, the other Families, or the NCR.
If the player character sides with Cachino, then the Courier will end up killing Nero as a result of the quest. Cachino then takes Nero's position as leader of the Omertas.
The Courier can lie to Big Sal and claim Nero ordered the Courier to take him out. Startled, Big Sal calls Nero a "backstabbing, two-timing motherfucker" and turns on him, saying "I knew this day would come."
Nero is also Caesar's puppet behind Operation: Racket, the operation for the Omertas to kill civilians on the Strip in order to take it over. If the player is working with Caesar's Legion and instead sides with the Omerta leaders, then Nero and Big Sal will instruct them to help Clanden prepare the chlorine bombs for the attack on the Strip and help Troike smuggle more guns into Gomorrah.
Attacking Nero will cause him and the rest of the Omertas to become hostile.