Leon Kowalski is a friend of Roy Batty. His emotions are at a much lower level of development than Roy's, but he evidently believes in Roy's quest for more life. Leon is classified mental level C. There is a debate about whether this means he is less intelligent compared to the average human, or just less intelligent compared to the average Nexus-6 Replicant.
He doesn't have the speed of thought Roy has when it comes to getting through a situation, resorting instead to simple violence.
Blade Runner Dave Holden is the first victim of this violence when he was just starting to suspect that his Voight-Kampff machine subject isn't just another human.
Leon later beats Deckard severely after he witnesses him kill Zhora. (It may be that Leon and Zhora had the first tentative steps to an emotional bonding.) Just as he is about to poke his fingers through Deckard's eyes, he is shot by Rachael, who has picked up Deckard's blaster.
Leon has cherished photographs of his friends. Unlike Rachael's false photos of her childhood, these are current photos of the only ones who mean something in his life.