Tatiana Evgenyevna Vyazemtseva was a KGB intelligence officer and member of Directorate S, based at the Rezidentura. First introduced in EST Men, by Season Four she is revealed to be secretly working with Department 12.
Background
For most of Season 3 Tatiana is shown to be a mid-level KGB intelligence office - she has her own office, yet must be read-in (given clearance) to partake in the more high-risk Rezidentura operations (e.g. Operation ZEPHYR). She seems to occupy Nina's old office.
Tatiana works with Department Twelve, the USSR’s covert ‘seventh floor’ germ warfare department - a project so secretive and politically controversial that even the Rezident Arkady does not know who, or what, it involves. His suspicions are aroused during Glanders - when he interrupts Tatiana in the radio room and she refuses to open the door. She apologizes and asks that he speak to Colonel Umametev at the Center.
Tatiana is in some capacity related to the supply-chain involving William’s samples being studied in the USSR. Because of her work in extracting Lassa virus from the Americans, she was offered the post of the Rezident in Kenya. She wanted her then-lover Oleg to be her deputy, but Oleg refused and ended up thwarting her plans to extract the virus through William. Later, she was demoted to her old post because of her failure.
In Season Six, she attempts to assassinate Fyodor Nesterenko as part of a plan to topple Gorbachev. As she does so, Tatiana is shot and killed by Elizabeth, who had been surveiling Nesterenko, suspecting his life was in danger.