Azuma Tokaku is one of twelve assassins sent to Myoujou Academy to kill a certain girl in their class. She soon opens up to her roommate Ichinose Haru, the girl who becomes targeted by the other assassins, and protects her from harm.
If the list is not daunting enough, see their official profiles and hear their voice samples here.
If the videos were eaten, try this Youtube search query.
The OP is Soushou Innocence performed by Uchida Maaya , and will be released 4/23/2014. The ending theme songs will be performed by a different girl each episode.