Who is Li Kui from Water Margin and what is their importance?

Li Kui is a prison guard who becomes mean when he gets drunk. He causes all sorts of problems for Song and Dai Zong because of his temper. Li travels to his home to save his mother and bring her to the Liangshan Marsh stronghold but she is eaten by tigers along the way. It is Li Kui who is responsible for poisoning the prefect's young son.