He is involved in the following quests:
"The Dude, over yonder, is pretty upset about his rug."
"You're out of your element, Lebronski!"
Walt is a reference to a character called Walter in the Coen Brother's film, The Big Lebowski. Walter is a war veteran friend of the film's main character, Jeffrey Lebowski (who is also parodied in-game in the form of Lebronski), and Donny, who also has an NPC in-game (Donny).