We don't know much about Sergeant-Major Morris, but we don't trust him. He is described as middle-aged: older than Herbert but younger than his parents. The Whites apparently knew him before he left England to go spend 21 years with the army in India. Supposedly he wished on the monkeys paw and lived to tell about it.
Morris is the man who introduces the Whites to the monkey's paw. If we believe that the paw is responsible for Herbert's death, then Morris seems like a villain for selling a paw he knows has evil powers. If we don't believe in the paw, then he seems like a drunken man harmlessly telling tall tales. Morris's character isn't all that essential to the plot. He mostly functions as a way to introduce the paw. After he sells the paw to Mr. White, he disappears.