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. In Mystery in the Forest it was revealed that Du has plans for how he would usurp various countries, but his plan for the Forest is in high gear because Tsai Su-yin is back in the game.