In Wii Sports, Dark Shiratori is the same as Shiratori. They are brothers. He only plays sports to replace his brother. He is the same level as Shiratori.
Wii Sports Resort
Wii Party U
He is nothing as Wii Party U will break if we try to rank him, like Shiratori. If you face him, good luck trying to find a new Wii U. Long story short, everything breaks when he is in Wii Party U. We just cannot rank him without breaking anything, so he is just Advanced Master.