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 15 battles with Gaara of the Desert versus any Konoha Ninja. (15/15) Win 15 battles with Temari versus any Konoha Ninja. (15/15) Win 15 battles with Kankuro versus any Konoha Ninja. (15/15)