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 5 battles with Gaara of the Desert versus Rock Lee or Uchiha Sasuke. (5/5) Win 4 battles in a row with Gaara of the Desert. (4/4) Win 5 battles with Temari versus Nara Shikamaru or Tenten. (5/5) Win 4 battles in a row with Temari. (4/4) Win 5 battles with Kankuro versus Aburame Shino or Sakon. (5/5) Win 4 battles in a row with Kankuro. (4/4)