EVA, also known as Tatyana (Russian: Татьяна), and later known as Matka Pluku, and Big Mama, was a spy who fought alongside Naked Snake during Operation Snake Eater. While posing as a KGB agent, she actually worked as a spy for the Chinese People's Liberation Army, tasked with obtaining the Philosophers' Legacy. She later became the surrogate mother of Solid Snake and Liquid Snake.

During Operation Snake Eater, EVA never revealed her real name to Naked Snake. When Snake asked her for it, she refused to tell him and asked "What's wrong with Tanya?" "Matka Pluku", one of two names she adopted during her anti-Patriot activities, the other being Big Mama, is a Czech phrase that means "Mother of the Regiment."

As a spy, EVA used her feminine charm to infiltrate Volgin's ranks. During Operation Snake Eater, EVA told Naked Snake "the least you can do is call me Cynthia"; a reference to the codename of Amy Elizabeth Thorpe, known as the most successful spy in history. She was active during World War II, and used her wit and charm to elicit countless secrets, even the Naval codes of French Vichy government. Snake also called her "a regular Mata Hari", a Dutch exotic-dancer during World War I who was accused of, and executed for, being a spy. As "Tatyana," she was also one of the few Groznyj Grad personnel to be allowed in Groznyj Grad's weapons lab's west wing in spite of not holding the rank of colonel.[13]

EVA's personal data as of 1964 revealed that: her eyesight was 20/16; her blood type was "A"; her favorite beverage was wine (she is seen drinking it with Naked Snake in a CIA safehouse after they returned to the United States); her measurements were 36-23-33; her favorite flowers were roses; her favorite jewellery were emeralds; and her ring size was 5½.[14] Her medical history reveals that she has had breast enhancements and proctitis.[15] Prior to posing as EVA, she received a wound near her breast, but refused to answer Naked Snake when he asked her how she got it. Besides the wound near her breast, she also received multiple burns, cuts, and other injuries because of Volgin's practicing his sadistic love of torture on her via "hentai play."[16] Snake, when discovering these injuries and that Volgin gave them to her, initially thought that Volgin did this to her because he found out she was the spy, though she was quick to remind him that if Volgin had found out about her role in aiding Snake he would have had her killed her on-the-spot instead of just torturing her; before explaining that it was simply done by him as part of his "hobby".[17]

Because of the origin of her "codename," EVA often referenced the Book of Genesis, such as: playfully asking if Snake had come to "seduce her" when they met; her monologue about her successfully seducing the snake and getting away with the forbidden fruit; her mention of her being exiled from China regarding her theft of the forbidden fruit (or more specifically, her failure to steal the real forbidden fruit); and her attempt to get "Adam" (Ocelot) to remember her while he was seemingly possessed by Liquid Snake by holding out an apple to him. Likewise, when she was leading the Paradise Lost Army resistance movement, she based their headquarters inside a church.

Despite her job as a female agent, as well as her personal admission to Naked Snake that she would fall in love "if it was required in her mission," EVA did genuinely love him. She would later volunteer to give birth to Big Boss' clones for Les Enfants Terribles. EVA thought of Solid Snake and Liquid Snake as her sons. She also didn't attempt to seduce people who were uninterested in her, or any woman for that matter, and felt uncomfortable having to explain it to someone who didn't understand why.[18]