J. F. Sebastian was a genetic designer employed at the Tyrell Corporation who suffered from Methuselah Syndrome, meaning his glands "grow old too fast," thus leading to accelerated aging and an early death. His condition caused him to fail passing the medical examination to leave Earth for the Off-world colonies, and so J.F. lived by himself in the Bradbury Apartments, given to him by Eldon Tyrell.
Sebastian was a gifted genetic designer, evident by his production of his "friends," equal part toy and organic. Sebastian worked with Tyrell to develop Nexus-6 replicants, specifically their nervous system. Sebastian also had a fondness for chess, playing solely with Tyrell and only managing to beat him once.
Encounter with Ray McCoy
In 2019, Blade Runner Ray McCoy, after having been rendered unconscious whilst pursuing leads at the Bradbury Building by Sadik and Clovis, return to the Bradbury Building and spoke with Sebastian about the deaths of fellow geneticists Marcus Eisenduller, Moraji, and the whereabouts of Luther and Lance.
Meeting the Replicants
Concurrently, six Nexus-6 replicants escaped the Off-world colonies and came to Earth to find a way to prolong their 4-year lifespans. Led by Roy Batty, these replicants began to infiltrate the Tyrell Corporation and interrogate scientists in order to glean information about replicant life span alteration.
Hannibal Chew, an eyeball manufacturer on DNA Row, revealed to Batty and Leon that Sebastian could take them to meet Tyrell in person. This prompted replicant Pris to stage a run-in with Sebastian, playing a lost, hungry, homeless person. Sebastian invited Pris into his home, allowing her to contact Batty and alerting him of Sebastian's location. After the deaths of Zhora and Leon, Batty met up with Pris at Sebastian's apartment in the Bradbury Building.
Sebastian and Roy
Sebastian was excited to discover that his new friends were Nexus-6 generation replicants, and his condition gave him something in common with them (namely accelerated aging). Batty brought this up, manipulated and then morally squeezed Sebastian into helping him get to Tyrell. The deciding factor in Roy's manipulation of Sebastian was a chess game in progress against Tyrell. The two opponents had chess boards in their homes; the pieces were arranged identically on both boards. Roy studied Sebastian's board and was able to discern two moves that would win the game for Sebastian.
Confronting Tyrell and death
After arriving at the Tyrell Corporation, Sebastian nervously relayed the winning movements to Tyrell, who granted him access to his living quarters. Upon meeting his "maker," Roy demanded an extension of his life and a brief discussion followed. Infuriated at Tyrell's inability (or unwillingness) to grant him longer life, Roy crushed Tyrell's skull and then murdered Sebastian before fleeing.
Behind the scenes
In the comic adaptation and novelization of the film, Sebastian is identified as being 20 years old rather than being 25 as mentioned in the film. This is consistent with the February 23, 1981 draft of the film's screenplay.
In the video game, if the player asks Sebastian too many questions or brings up anything regarding replicant Nexus-6 information, Sebastian will realize McCoy is a Blade Runner and ask him to leave, refusing to answer further questions. One of the four DNA data chips on furthering replicant lifespans can be found on Sebastian's computer, which must be turned on and resumed in order to collect.