J. F. Sebastian seems to lead a strange and lonely existence. He's a genetic designer for the Tyrell Corporation—responsible for the creation of the rebellious replicants—and he lives in an apartment with a giant number of beings that are half-living and half-toy. After meeting Pris and letting her stay at his apartment, he tells her: "They're my friends. I make them." In a way, he's a prophetic symbol: he's like people in the present day who only have friends from virtual worlds online and who date imaginary characters and things like that. This makes him both sad and sympathetic.
Although Sebastian seems attracted to Pris after he first meets her, this doesn't really go anywhere, given that Roy shortly shows up. Roy forces Sebastian to take him to Tyrell, which he does. But things quickly go south: Tyrell says he can't extend Roy's lifespan, which causes Roy to murder Tyrell and then kill Sebastian off screen (we find out about this when Bryant radios the info to Deckard).
Additionally, Sebastian suffers from a disease called "Methuselah Syndrome" (named after the oldest man in the Bible), which causes him to age prematurely. The replicants play on this in order to win his sympathy, pointing out that they too suffer from "accelerated decrepitude."
Again, what's the real difference between the humans and the replicants here? Even humans can be made with short lifespans, and we're guessing they'd also like to know exactly what it has to be that way. It's hard not to feel sympathy for these creatures.