The World Has Already Ended.

Following Rentaro Satomi's defeat of the Stage Five Gastrea, Tokyo Area returns to uneasy normalcy. When the ambitious leader of Osaka Area, Sougen Saitake, demands a visit with Lady Seitenshi, she needs a new bodyguard--and who should she pick but the city's new hero, Rentaro. However, dark forces are conspiring against Lady Seitenshi, and their origin is closer to home than anyone dares to imagine. Rentaro and Enju will have to confront a new kind of threat in order to protect the tenuous peace they've won--but how do you fight an enemy you can't even see?