The Passenger aka Nadia Santos
Nadia is the daughter resulting from an affair between Irina Derevko and Arvin Sloane. She is the half-sister of Sydney Bristow. According to Irina's sister Katya, Nadia was born in a Soviet prison in 1982 and was taken away from her mother the day after her birth. Identified by the Soviets as "The Passenger," a being with a "direct conduit" to Milo Rambaldi, Nadia spent the first few years of her life as the subject of experimentation, including injection with a "Rambaldi fluid" which triggered her connection to Rambaldi.The main culprit in her torture was Dr. Zhang Lee
Nadia is being sought by her father, Arvin Sloane, who wants to use her connection to Rambaldi to locate an artifact called the Sphere of Life. This artifact is said to house Rambaldi's "consciousness". The Convenant captures her and injects her with the same Rambaldi fluid she was subjected to as a child. This triggers a "muscle memory," causing Nadia to write out a complex algebraic equation which, when solved, reveals the coordinates of the Sphere of Life. Sydney Bristow, Michael Vaughn and a CIA team then recover Nadia. Later, Arvin Sloane approaches her and convinces her to join him on a quest for the Sphere of Life. Nadia says she had moments of lucidity as the fluid wore off and in those times she altered the equation so only she knows the true location of the Sphere.
Nadia and Arvin Sloane locate the Sphere of Life in Siena. Nadia, seeing the madness Rambaldi inspires in her father grow as they get closer to the artifact, and after having apocalyptic visions upon touching the Sphere, she refuses to retrieve it. The vision was of "The Passenger" and "The Chosen One" (Sydney Bristow) fighting to the death.
When blood red horses wander the streets and angels fall from the sky, the Chosen One and the Passenger will clash....and only one will survive
APO head to Sovogda to stop Elena Derevko and Arvin Sloane from implementing their endgame by activating a huge Mueller device over the streets of Russia.Elena Derevko injects Nadia with a compound which turned her into a mindless killing machine.The Passenger and The Chosen One fight,but Nadia is shot down by Arvin Sloane.She survived but slips into a coma where she will stay for one year.Racked with guilt over his involvement in his daughters condition,Arvin Sloane vows to do anything in his power to find a cure,even betraying APO,who have only just accepted him back.
With his obligations to Prophet 5 complete, the organization gives Sloane a hypodermic needle containing a Rambaldi-designed drug that will cure Nadia's comatose condition. Nadia awakes from her coma,but is accidentally killed by her father a few hours later.
The Rambaldi prophecy about the the Chosen One and the Passenger did come true.It stated that the the Chosen One and the Passanger would clash and only one would survive.It didn't say that she would die there and then,but she would die eventually.
There was some confusion in season 3 and 4 about who rescued Nadia from the KGB and who took her to Elena Derevko's orphanage.The whole story about Bill Vaughn rescuing her was completely false.It was a ploy by Elena Derevko to acquire a Rambaldi device.