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 Inuzuka Kiba versus any Mist Ninja. (6/6) Win 10 battles with Hyuuga Hinata versus any Sand Ninja. (10/10) Win 15 battles with Aburame Shino versus any Sound Ninja. (15/15)