"So you're the hero everyone's been talking about. Perhaps you could help a poor, simple farmer out of a terrible mess? At the edge of my orchard, just south of here, there's a horrible thing swelling out of the ground! I can't get to my crops or my bales of hay, and my poor cows will starve."— Lester(src)
Lester the Farmer was an inhabitant of Tristram.
During the Darkening of Tristram, Lester found something swelling out of the ground, preventing him from getting to his crops and bales of hay. After seeing Adria, she gave him a rune bomb, saying it would remove the thing from the field. He asked a hero to do it for him, as he had to watch his cows. The hero obliged and detonated the bomb, revealing the entrance to the Festering Nest.
Lester is exclusive to Diablo: Hellfire. He can be seen talking to his cows, and the other townsfolk think he has gone mad, but feel sorry for him because he lost everything. He initiates the Farmer's Orchard quest. Unlike the classic Diablo NPCs, Lester does not have any gossip and can not be asked about quests. After the hero opens up the Hive with the rune bomb, the next time he/she returns to Tristram from underground, Lester will have departed.
In beta versions of Hellfire, Lester used to have a bigger role as a comic relief character. Using the Command.txt exploit replaces Lester with the Complete Nut (Lester in disguise), allowing players to take the Jersey's Jersey quest. It's possible that, originally, Lester would remain on the map as Complete Nut rather than disappear after his quests.