J. F. Sebastian/John R. Isidore is a genetic designer, working for Tyrell/Rosen.
He is stuck on Earth due to his medical condition, "Methuselah Syndrome," which also gives him something in common with the Nexus 6 Replicants, namely accelerated aging. With the Bradbury Building all to himself, he makes the most of his gifts, even making his own Toy-friends.
He falls easy prey to Pris. Sexual tensions arise when Roy Batty shows up. Although J.F. is excited to discover that his new friends are Nexus-6 generation replicants, he finds his life rapidly going downhill shortly afterward when he is manipulated and then morally squeezed into helping Roy Batty get to Tyrell. During Rick Deckard's drive to the Bradbury Building it is heard on the radio that his body was found with Tyrell's.
The deciding factor in Roy's manipulation of Sebastian is a chess game Sebastian is playing against Tyrell. The two opponents have chess boards in their own homes; the pieces are arranged identically on both boards. Roy is seen studying J.F.'s board and is able to discern two moves that will win the game for J.F. After traveling to the highly secure Tyrell Corp. building, J.F. nervously relays the winning movements to Dr. Tyrell, who grants him access to his living quarters. Upon meeting his "maker", Roy demands an extension of his life and a brief discussion follows. Infuriated at Tyrell's inability (or unwillingness) to grant him longer life, Roy kills Tyrell and then Sebastian before fleeing.
Deckard receives news of Sebastian's death from his chief, Bryant, and pursues the lead to Sebastian's home.