Who is Master Zheng from Water Margin and what is their importance?

Master Zheng is also called the "Lord of the West." He hopes to buy the girl, Jade Lotus, as a concubine and promises to pay her father three thousand strings of cash for her. He never pays Jade Lotus' father the money. When Zheng's wife drives the girl out of the house, Zheng demands her father return the money, which Zheng never paid. Lu Da gets involved in the affair and accidentally kills Zheng trying to make the matter right.