In Blade Runner, the Nexus series of replicants – genetically engineered by the Tyrell Corporation – are virtually identical to an adult human being, but have superior strength, agility, a wider temperature range tolerance, and variable intelligence depending on the model. The Nexus-6 series is said to have been made for superior strength and agility, for use in off-world mining colonies.
Because of their physical similarity to humans a replicant must be detected by its lack of emotional responses and empathy to questions posed in a Voight-Kampff machine test. A derogatory term for replicant is "skin-job" (in the original release, the Deckard voiceover explicitly states this--in the subsequent releases without the voiceover, the viewer must infer this).
As of 2019, the latest model of replicant is almost indistinguishable from humans. In fact, the Tyrell Corporation advertises this model as "More human than human." If replicants are given memories (and thus think that they are indeed human), the VK test almost fails- It takes over a hundred questions to expose the replicant known as Rachel.
In the film, the character Chew is revealed as the Tyrell Corporation employee who designed the eyes for Nexus-6 replicants Roy and Leon. They both hunt him down and demand to know how to enter Tyrell's inner sanctum.
Roy Batty, perhaps the most perfect of this series, and the one who led the rebellion of replicants back to Earth, said he had done "Questionable things." His creator, Tyrell, replied that he had done "extraordinary things."
Like all replicants, they had a limited lifespan, the Nexus-6 being genetically designed to have a very short lifespan of just a few years. Batty and the other replicants returned to Earth to seek more life, but Tyrell said he was unable to alter the genetic make-up of the series.
In the game McCoy states how Nexus 5's and 4's would normally just give up when cornered, but 6's never stop fighting to stay alive.
As he sat dying with Deckard on a rooftop, Batty said of the uniqueness of the Nexus-6 Series and their ability to go where no humans had gone, and do things no human had done.
"I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion; I watched C-beams glitter in the dark near the Tannhauser Gate. All those moments will be lost in time like tears in rain."