Viper is the name of Zero's syndicate and the primary antagonist faction of Red vs. Blue: Zero. They are a trio that seeks the Ultimate Power.
Prior to the start of Red vs. Blue: Zero, Agent Zero, originally designated Agent One, was part of GLASS, a special ops program similar to Project Freelancer working under the Alliance of Defense, as well as a friend and teammate of Axel. There he witnessed brutal experimentation at the hands of Starlight Laboratories, including the experiment that saved the life of Phase and gave her the ability to use a holo echo. Zero was promised to be the new leader of GLASS's new team, Shatter Squad, but the Great War had ended and the project was disbanded. Feeling cheated out of his chance, Zero abandoned the AOD and recruited Phase and Diesel to seek out the Ultimate Power for his own agenda.
In order to access the temple containing the Ultimate Power, Viper needed to obtain a number of Great Keys needed to open the temple. They started with stealing an artifact at a hidden facility, Falcon Base, where the artifact was being transported by Washington and Carolina. Viper attacked the base, stealing the artifact, injuring Carolina and severely wounding Washington.
Viper continued collecting keys, including the one belonging to Lavernius Tucker, and accessed the temple containing the Ultimate Power. While Zero went on to obtain it, Phase confronted her father, Agent West, and Diesel battled Carolina. Carolina defeated Diesel, killing him with sticky grenades. Phase defeated West, but spared his life and defected from Viper to help Shatter Squad take down Zero, who had been overpowered by the Ultimate Power. Sensing weakness but also potential in Zero, the armor's alien construct Black Lotus absorbs Zero and transports them both away, leaving Viper disbanded.