Du is a descendant of the original (in the context of the Stewards anyway) usurper of the Craneslake Heights. He tried very hard to finish usurping the throne there, but seems to have given it up after Return of the King. In this context, Xiao Fa consulted with Dragon (regarding whether to use Du as part of his own ritual) and adjudged Du to be "Spider's Pawn".
His concept is Usurper. He should be considered armed and dangerous, especially if there's usurping to be done.