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

Song originally works as a clerk in Yuncheng. He was born in the Song Family Village. He has been given the nickname of Dark Song Jiang because he is short and swarthy in appearance. Even as a clerk, Song was well-known for helping anyone who requested his help.

Song becomes an outlaw when he attempts to keep his friend, Chao Gai, from being arrested for his participation in the robbery of Premier Cai's birthday gifts. Song's wife, Poxi, manages to get a letter that Chao has written to Song thanking him for helping him avoid arrest in connection with the gift robbery.