Pre-Virtuous Mission & Snake Eater
The Boss founded the Cobra Unit during World War II. However, the Cobra Unit disbanded following the war. The Boss and her lover, The Sorrow had a baby during World War II on the battlegrounds in Europe. They named him Adam, who would later be known as Ocelot. However, this was only hinted in Metal Gear Solid 3 and was never told directly so it's left for fans to speculate as they wish. Unfortunately, a member of the Philosophers abducted Adam and she didn't see him for 20 years. However, the Philosophers took her baby away after literally giving birth on the battlefield.
The Boss would later join the SAS and serve alongside Zero. She would go on and train spies in China's charm schools (where she was also an instructor of Eva). The Boss is also the mentor of Naked Snake (later known as Big Boss), she trained him for ten years and even lived with him. The two had a motherly and son relationship which is something she didn't get to have with Adamska. The Boss and Naked Snake developed CQC during their time together. She fought during the Korean War. In 1951, she was at a nuclear test in Nevada. There, she was exposed to radiation which lead to her being chosen for the Mercury Project in 1960. They chose her because no technology existed at the time to protect people from space radiation, and The Boss' previous exposure to radiation (and surviving) made her a prime candidate. Seeing Earth from space inspired her to one day unite the world as one. In 1962 the Philosophers ordered her to assassinate her former lover, The Sorrow. She would go through with it and killed him in battle as it was a matter of life and death.
The Virtuous Mission & Snake Eater
On August 24, 1964, The Boss assisted Naked Snake during his attempt to rescue Sokolov. However, Naked Snake and Zero lost contact with The Boss during the mission. Naked Snake would meet up with The Boss after he successfully found Sokolov. On the bridge, The Boss revealed that she had defected to the Soviet Union, rejoining the remaining members of the Cobra Unit, and gave two miniature nuclear missiles to Colonel Volgin. The Boss then broke Naked Snake's arm, and threw him off the bridge. As they flew away in their helicopter, Colonel Volgin fires one of the nuclear missiles on his own people. This caused the Soviet Union to blame the United States for the attack, to which the U.S. passed the blame onto a single "traitor," The Boss.
The U.S. then launched "Operation Snake Eater," and had Naked Snake infiltrate Russia and eliminate The Boss to clear the U.S.'s name. It turned out that The Boss was acting as a double agent, and had not really defected. Her mission was to acquire a microchip that contained the location of the Philosopher's Legacy. However, after Colonel Volgin fired the nuclear missile, she had to die in order to pacify the Russians preventing another world war. And The Boss was ready to make that sacrifice for her country. While Naked Snake fought Colonel Volgin, The Boss took Eva out of the area and told her everything so that she can in turn, tell Naked Snake the truth as The Boss was also forbidden to tell Naked Snake the truth. Later, The Boss fought against Naked Snake in an emotional battle which ended with Naked Snake killing The Boss with her own Patriot gun.
Her legacy lived on and inspired the creation for The Patriots. Her desire for a united world free from conflict and the pressure of the cold war inadvertently resulted in the patriot's conquest and restructuring of the world through digital means. The A.I. systems controlling the world fifty years after her death didn't understand the need for free will and choice in her vision for the future.
The Boss was voiced by Lori Alan in all the MGS games she's currently appeared in.