The B Rank Missions are only assigned to to experienced ninja. With these missions you are able to unlock villians and subordinates of the most dangerous ninjas.
Win 4 battles in a row with Kidoumaru. (4/4) Win 4 battles in a row with Sakon. (4/4) Win 4 battles in a row with Tayuya. (4/4) Win 4 battles in a row with Jiroubou. (4/4)