This is a good question, because Sergeant Major Morris is an ambiguous figure. In the words, we cannot tell whether he is a good or sinister man. As for a description, there is not much.
What we know about him is that he is well travelled. As a youth he went out and travelled. He saw a lot around the world. He spoke of wars, plagues, and various nations. Within this context, he also recounted a story of a mysterious monkey's paw, which was supposed to grant wishes but with horrible consequences. The person who gave it to Morris wished for death as his last wish.
The fact that Morris did not destroy the paw leads the reader to believe that he is an evil man. To be sure he throws the paw into the fire, but he allows Mr. White to take it out. Moreover, he tells Mr. White exactly how to use it, knowing fully that it will cause his demise.
In conclusion, Morris at best is a well-travelled man with little concern for others, or he is an sinister man who set up the Whites for suffering.