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 6 battles with any Ninja of Team 7 (Naruto, Sakura, Sasuke). (6/6) Win 6 battles with any Ninja of Team 8 (Hinata, Kiba, Shino). (6/6) Win 6 battles with any Ninja of Team 9 (Lee, Neji, Tenten). (6/6) Win 6 battles with any Ninja of Team 10 (Chouji, Ino, Shikamaru). (6/6) Win 6 battles with any Ninja of the Sand Genin (Gaara, Kankuro, Temari). (6/6) Win 6 battles with any Ninja of the Sound Genin (Dosu, Kin, Zaku). (6/6)