During the Voight-Kampff 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 asking many more questions than the average replicant, Tyrell asked Rachael to leave him alone with Deckard. As Rachael left, Tyrell revealed to Deckard that she is a prototype model of a different kind of replicants Tyrell has interest in constructing. Tyrell also informed Deckard that Rachael is unaware that she is a replicant and that he would like Rachael to remain aware of her state.

Rachel Blade Runner