There is a problem with his more recent, advanced Nexus-6 Replicants - they start to develop their own emotions. The other Replicants we see provide plenty of evidence that this is true. They then start to rebel against being slaves. Tyrell believes that this is because they have no framework within which to deal with their new emotions. Thus, if he can just gift them with memories, he hopes that this will help them cope better and therefore be a "better product."
Rachael is created, but more than just having implanted memories of Tyrell's niece, she is led to believe that she is human. We do not know how long she has been "switched on", but Tyrell admits that he thinks she was "beginning to suspect." After imprinted with false memories, Rachael was a secreatry for the Tyrell Corporation and assisted Tyrell in running operations.
Voight-Kampff Testing and Meeting DeckardEdit
Deckard was sent to the Tyrell Corporation building to use the Voight-Kampff machine and test any replicant present within the complex. Being aware that a blade runner is visiting the complex, Rachael entered the meeting room of the Tyrell Corporation Building and greeted Deckard after commenting on how the Corporation's owl is artificial, similar to majority of the owls within the city.
Believing she is human, Rachael asked Deckard if he has ever retired a human by mistake. After Deckard replied that he has not, Rachael replies back saying that in blade runners' line of work, it is always a possibility. However, the conversation was cut short when Tyrell entered the room and requested that he wanted to see the Voight-Kampff Test for himself on a human subject. Trying to convince Deckard that Rachael his human, Tyrell insisted Rachael to be used as the human test subject.
During the test, Deckard noticed Rachael's eye patterns are of a replicant and realized that Rachael is an experimental replicant with implanted memories which provide a cushion for her emotions. After the test, Deckard left the corporation after informing Rachael that she is a replicant and her memories are false
Reconciling with DeckardEdit
After the Voight-Kampff Test, Rachael felt uneasy and was questioning her humanity claiming that she is human and her memories are her own. The next evening, Rachael left the corporation and went to Deckard's apartment.
Rachael implored with Deckard that she is human until Deckard explained to her that her memories are false due to its connections with Tyrell's niece. Shocked by the evidence found, Rachael began to cry in silence and left Deckard's apartment.
Rachael, after learning the truth of herself, accepts her condition and continues to live life the way she sees fit. One night while walking alone through the city, Rachael hears a fight near her and decides to investigate.
Getting close enough to see the physical confrontation, Rachael notices that the two men fighting are Deckard and Leon. Rachael notices Deckard's gun on the ground as it was knocked off of Deckard's hand during the fight. Rachael picks it up and shoots Leon moments before Leon was about to kill Deckard.
Loss and LoveEdit
After saving Deckard's life, Rachael goes back to Deckard's apartment to stay with him after realizing that she has nothing much to live on for due to her short lifespan as a replicant.
While at Deckard's apartment, Rachael unties her hair and changes her clothes to wear something more comfortable and less professional. Rachael sits on the stool next to Deckard's piano and begins to play a tune on the piano.
After Deckard questions her ability to play, Rachael explains that she has fond memories of learning how to play and playing for others. Rachael then ponders on her life and realize that she is now able to live her life free from Tyrell and blade runners as both Rachael and Deckard has fallen in love with each other.
Rachael and Deckard begin to get intimate as they kiss one another and get undressed before having sex. The next morning, Deckard leaves his apartment to report in before being sent to Sebastian's apartment to locate another replicant.
Leaving the Life BehindEdit
After the death of Roy Batty, Deckard returns to his apartment and cautiously enters when he sees the door is ajar. He finds Rachael alive and informs her that they must leave the city.
As they leave Deckard comes across an origami calling card (an origami figure of a unicorn) left by Gaff to show that he has been at the apartment while Rachael was asleep.
Both of them are allowed to live - Deckard by Roy, and Rachael by Gaff. After Deckard examines the origami figure, Deckard suddenly questions his humanity while Rachael watches by the exit door. After turning towards Rachael, both Deckard and Rachael depart toward an uncertain future together.
In the original theatrical edition of the film, Deckard's voice over as they drive north suggests that she may have a longer lifespan than any of the other Nexus-6s that were created.
A Life with DeckardEdit
Months after leaving the city, Rachael and Deckard live together on the outskirts of Los Angeles. Rachael, however, spends majority of her new live in a cryotube to extend her lifespan and only periodically exits the cryotube.