Black Viper is the main villain and final boss in Operation C and Contra 4. According to the manual for Contra 4, "she" is the "sister" of Gava.
Black Viper initially comes to Earth and takes over an unnamed nation, using it's brainwashed military forces to prepare to take over the world. The government sends the Contras Mad Dog and Scorpion to fight them. The two invade the nation and fight through their defenses, defeating the cyborg who protected Black Viper, only to find that Black Viper was just a large cell biomass inside a liquid vial. They destroy the vial, putting and end to Black Viper's plans for the time being.
Two years after the Alien Wars, Black Viper returns, this time controlling the Red Falcon forces, starting a new attack upon Earth. After the government picks up a signal on the Galuga Archipelago, the place where the alien invasion originally started, the send in the Contras Bill Rizer and Lance Bean together with Mad Dog and Scorpion to put and end to the alien threat once and for all. Black Viper is found on the Harvest Yard, having evolved into a mantis-like monster.
Upon it's initial defeat, Black Viper buries herself into a pile of corpses, reemerging as a colossal alien. The contras blow up through the monster's back, climbing through it's insides until they reach the brain for the final battle above the Earth's atmosphere. The monster finally dies upon having it's brain destroyed.