Who is Wu Song from Water Margin and what is their importance?

Wu Song meets Song Jiang at Chai's manor. Because he kills the tiger that kills people on Jingyang Ridge, he is made a constable. He meets with his brother, Wu the Elder, and moves in with him, but his brother's wife tries to seduce him and he leaves. Wu Song later learns that Golden Lotus has killed his brother. He discovers the truth about Wu the Elder's death and avenges his murder.