Roy Batty is the leader of the renegade Nexus-6 replicants. He is very intelligent, fast, and skilled at combat, and yet still learning how to deal with developing emotions. He leads a few of his fellow replicants on what is inevitably a fruitless search for more life. As hope slowly fades away and his friends are eliminated one by one, it is his experience that brings up the question of "What is human?"
The "prodigal son" of Eldon Tyrell, Roy returns to his "father" by hijacking an off-world shuttle with the assistance of some other replicants (Leon, Pris, Zhora, and two others). They then kill the crew and set a course for Earth. First seeking a longer life, then asking forgiveness, he ultimately destroys his own maker.
After a failed attempt to break into Tyrell's home and company headquarters (which results in the deaths of the two unnamed replicants), Roy and Leon made investigations into ways that they could lengthen their lifespans by viciously interrogating Hannibal Chew. Chew directs them to J.F. Sebastian, and Pris goes to meet J.F. first. Later on that day, Zhora is killed by Blade Runner Rick Deckard and Leon is killed by Rachel.
Roy arrives the next morning at J.F.'s home, the abandoned Bradbury Building. He tells Pris about Leon and Zhora's deaths, and is overwhelmed by emotion and barely able to tell Pris. He helps J.F. win a chess game against Tyrell after J.F. tells him that Tyrell will let him go to meet him in his home if he is able to beat him in chess. Roy and J.F. go to meet Tyrell, and Tyrell and Roy briefly debate the possibility of extending the lives of replicants, and Tyrell concludes that it is impossible. Roy asks Tyrell for forgiveness of his sins, but Tyrell brushes this off by telling him that he's also done wondrous things, which does nothing to comfort Roy. Roy then kills Tyrell, and then kills J.F. before retreating back to the Bradbury Building.
There is no future for Roy, as his friends are killed and his hope for more life is smashed. Deckard arrives at the Bradbury Building and "retires" Pris. Roy begins to play a cat-and-mouse-game with Deckard, frightening and taunting him until he finds Pris's body, and he breaks down into tears. Roy begins to taunt Deckard more violently, breaking two of his fingers as vengeance for Zhora and Pris, then encourages Deckard to join him in fits of violence after Deckard hits him with a pipe. Roy then forces Deckard to retreat to the roof of the Bradbury Building.
On the roof, Deckard attempts to escape Roy by jumping to another building, but fails due to his wounds and also because the rain caused him to slip. Deckard is now trapped, hanging onto the roof as his hands slip and he is about to fall to certain doom. However, Roy then performs an act suggesting perhaps that he has gained the empathy that is the thin dividing line between the Humans and the Replicants: he grabs his adversary & helps him up. Moments later, as his four-year lifespan draws to a close, Roy reflects on events of his life such as fighting at Tannhauser Gate, and then dies calmly.
Showing more "humanity" than the men who seek to kill him, we are left wondering at what exactly makes us human.