Who is Mary Starbuck from Ahab's Wife, or, the Star-gazer: A Novel and what is their importance?

Mary Starbuck is the wife of one of Ahab's crew members. She becomes a good friend of Una's after Ahab loses his leg and suggests that Una and Justice move to Siasconset in the house next door to her. They mourn the loss of their husbands together. Mary eventually marries Isaac and moves to the Sankaty lighthouse.