Who is Lao Dah from Gladys Aylward: The Little Woman and what is their importance?

Lao Dah is the leader of a groups of bandits. He accuses Gladys of spying on him, and when during a service Gladys proclaims that "we are all sinners," Lao Dah takes it personally and thinks Gladys knows that he himself is a sinner. After the misunderstanding is cleared up, Lao Dah becomes curious about Christianity and eventually becomes a Christian, renouncing his former ways.