Paz Ortega Andrade is the true main antagonist from "Metal Gear Solid: Peace Walker" and turned anti-hero in "Metal Gear Solid V: Ground Zeroes".
Metal Gear Solid: Peace Walker
Her real name is Pacifica Ocean and she was a Cipher agent who infiltrated the Militaires Sans Frontièrs during the Peace Walker Incident in 1974. She had taken possession of Metal Gear ZEKE and she gave Big Boss the opportunity to surrender to Cipher. But he resists and after a hard fight, Big Boss comes out the victor and Paz ends in the sea.
Metal Gear Solid V: Ground Zeroes
Big Boss finds out that Paz was still alive and a prisoner of XOF, he goes to save both her and Chico. Once you get on the helicopter, he removes a bomb from the belly of Paz, placed by Skull Face to kill Big Boss. However, you will find that Paz had another bomb and saves Big Boss by throwing herself from the helicopter. Before her rescue, Skull Face had promised to Paz that Big Boss would be safe if she would tell him where Zero is, and so, she confessed.
Metal Gear Solid V: The Phantom Pain
Paz appears as an illusion of Venom Snake.