Who is Daniel "Spud" Murphy from Trainspotting and what is their importance?

Spud is a fellow addict who spends his time with the group that includes Mark Renton and Frank Begbie. Spud is a bit slower than his compatriots and not as charming. When he is using, Spud is almost unintelligible. He is a good soul, however, insistent that people are put on earth to be kind. He insists on kindness to animals and shows deep empathy for Lesley when her baby dies. Renton considers them best mates. When Spud first appears in the novel, he is passed out after several days of drinking around New Years.