I thought that Yojinbo could be Shadow.The reasons of my thoughts are that:
1.They both hiding their faces (Mask).
2.They do anything for money.
3.The Dog!!! on their side.
4.Shadows`s a Ninja and Yojinbo uses Ninja skills like Wakizashi.
So it could be that Shadow is Yojinbo?!?Why not!?!