If you are dissatisfied with the decision of the Patent Reexamination Board, you will file a lawsuit with the People's Court against the Patent Reexamination Board.
The request for invalidation is filed with the Patent Reexamination Board, and the other party is the patent being requested to be invalidated. The patentee is the patentee, but in most cases it is the agent entrusted by the patentee to defend and participate in the oral hearing.