From MangaHelpers:

Orihara Rentarou thinks that he's an ordinary high-school student, who spars everyday with his much more highly skilled friend Haruhiko, who he thinks of as an older brother. His school is near a blocked-off area of town, where people are forbidden to go due to the presence of "Others," monsters who attack humans. Then one day, after bats mysteriously swarm the school and seem to target him, he begins to hear a voice in his head that tells him to go to the blocked-off area.

Soon he discovers that the Others are real, and that he is one of them. There is a group called "Samurai," whose members are Others dedicated to fighting the human-killing monsters who are contained in the area. After this revelation, Rentarou is uncertain of who he is anymore, but he joins Samurai to fight evil Others alongside Haruhiko and their other comrades. [tethysdust]