Roy Batty is the leader of the renegade Nexus-6 replicants and the main antagonist of the film. He is highly intelligent, fast, and skilled at combat, and yet still learning how to deal with developing emotions.
With an A Physical Level (superhuman strength & endurance) and an A Mental Level (genius-level intellect), he is probably the most dangerous of all the fugitive replicants. He is a combat model, used off-world for military service. He and five other replicants come to Earth hoping to find a way to lengthen their lifespan.
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. Time to die.
Blade Runner // Batty
Video: I've seen things...
Roy Batty is able to use J.F. Sebastian to get a meeting with Tyrell, the head founder of the company and his creator. Tyrell refers to him as his "prodigal son", and tells him his life cannot be extended, but that he should revel in the life that he has, as he has done and seen things others could only dream of. Following this, he kills Tyrell and probably Sebastian.
Deckard retires the remaining replicants and is hunted by a dying Roy. Trying to escape, Deckard ends up dangling from a building and is saved from the fall by Roy. As he dies, Roy tells Deckard about the things he saw in his life and how all those memories would be gone forever. He then smiles, saying, "Time… to die", and passes away.