So. This is what’s really happening in Blade Runner.
Tyrell Corp have a mission to create Replicants that are “more human, than human”.
To this end Tyrell have made a perfectly human Replicant. He has only human strength. He will age live a normal human lifespan. He will be able to father children.
This Replicant is Deckard.
But to be truly human this Replicant must become capable of empathy and love.
So a second human Replicant is made. Also perfectly human, able to mother a child, but yet to develop empathy.
This Replicant is Rachel.
Deckard is given the memories of an LAPD detective named Gaff. These memories include a fantasy unicorn. Gaff makes a paper unicorn and leaves it for Deckard as a clue to his real identity.
Deckard has just been activated by Gaff when we first see him reading the newspaper then ordering noodles. Deckard is to be given a mission to hunt the rogue Replicants.
Deckard is being placed within a story that will teach him empathy as he hunts the Replicants. His hardboiled detective persona is just a character in this story. Deckard’s apartment is Gaff’s apartment.
Deckard is sent to Tyrell Corp as a setup to meet Rachel. They have been made as each other’s perfect mate. As planned they fall in love, and will later have the first Replicant child.
Deckard learns the lesson of empathy, and learns he is a Replicant from Gaff’s unicorn. Gaff’s job is to bring Deckard and Rachel back in. However, Gaff realises that Deckard, who has all his memories and fantasies, is him.
Tyrell has also misjudged his plan. The rogue Replicants were able to find and kill their “father”. So without Tyrell, Gaff is free to let Deckard and Rachel go.
Some decades later the story of Deckard and Rachel’s child is told in Blade Runner 2049.
So. Yes, Deckard is a Replicant.
But. The Deckard Replicant is “more human, than human”.
A very Philip K Dickian final twist, to tell a story about human empathy. Watch the full video essay to see how Blade Runner is an empathy test…for the audience.
Damien Walter 
The biggest misunderstanding about replicants is that they are artificial; robots or something. If you watch the film, there’s unending references to genetics. They’re not robots or synthetics…they’re gene altered clones. Their strength is increased biologically, just as their lifespan is shortened.
Roy seems child like because he didn’t have a childhood, but was grown in adult form, given training, and sent to work. He behaves emotionally like a three year old, because he only has three years of emotional development. The memory implants were intended to soften this, giving them more “life experience” even though artificial.
Point being, empathy is a brain function. People with fully biologically normally developed brains have empathy. It’s not so much a learned behavior, but the result of certain lobes on the brain reaching full development.
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