Leon Kowalski is one of four surviving escaped replicants to make it to earth and seek to extend his life span in the 1982 cult classic film Blade Runner based on the 1968 speculative science fiction novel Do Androids Dream of Electric Sheep by Philip K. Dick. Leon is a genetically engineered human being created as a worker unit to do hard labor on off world colonies. Because he is a replicant, he was designed with a life span of only 4 years, and has already lived at least 2 at the time that he has come to Earth. Like the other replicants in the film, he is desperate to find a way to live longer than his built in expiration date dictates, and to avoid being caught by Blade runners, the police branch created specifically to find and execute rogue replicants.
Leon is first seen in the film taking a Voight-Kampff test, which is a test designed to gauge whether the taker is a replicant or not. Leon is clearly frustrated by the test and its questions, and frequently interrupts the test proctor. When prompted to answer a question about his mother, the nature of which a replicant could not honestly answer, Leon pulls out a pistol and shoots the proctor.
He is next seen with replicant leader Roy Batty, breaking in to a bioengineering facility, where eyes for replicants like themselves are made. The facility itself needs to be refrigerated to a point where it is hazardous for normal human beings without the use of insulated environmental protection gear. Leon and Roy come across a worker in the facility who claims to know nothing except for the design of eyes. This answer dissapoints Roy. Leon, at Roy's request, removes the worker's protective gear, leaving him exposed to the elements and clearly freezing. Through further intimidation, Roy and Leon force the man to surrender information about how to learn more about their design and who to seek out. The man tells them that they need to find J.F. Sebastian. The two replicants then leave the man exposed to the elements, presumably to die.
Leon finally attacks the movie's protagonist Rick Deckard, after Deckard has just killed fleeing rogue replicant Zhora, one of the only people who Leon cared about. Leon asks Deckard how much longer he has to live. When Deckard states that he only has a four year life span, Leon angrily retorts, "Longer than you!" Leon then proceeds to savagely beat Deckard. Leon's physical strength is beyond that of most humans including Deckard, and Deckard is clearly outmatched, but he is rescued by Rachel, a recently escaped replicant who used to work for Eldon Tyrell, the original designer of the replicants. Rachel shoots Leon in the head, kiling him instantly.
- Though it is never explicitly stated, there is speculation that Leon is in love with Zhora, another rogue replicant.
- Leon is the only replicant who Roy does not mention, and break one of Rick Deckard's fingers for, during the battle at the climax of the film. It is unknown whether he was not mentioned because Roy did not care as much about Leon, or because it was Rachel and not Deckard who actually killed Leon.
- Leon's intellgence is listed as class C. It is unknown if this means his intellect is average or below average in comparison to non-replicants.