Who is Lorena from Lonesome Dove and what is their importance?

At the age of seventeen, and with both her parents dead, Lorena met Mosby, a Southern gentleman. He had a fine pair of horses, and having nowhere else to go, she went with him. She sort of fell into a sporting kind of lifestyle purely by accident, beginning with her father when he became delirious near the end, and then Mosby who claimed to want to marry her at first, but then took her to Gladewater where he used her at his will, threatening to send the dogs after her if she ran. Constantly bothering her with his "radish", wanting a "poke", Lorena became so depressed she hardly spoke. Mosby gambled, and when he lost, he would offer Lorena for a poke to square his debt.