A 2009-current light novel series by Kawahara Reki, with illustrations by HiMA, serialized in ASCII Media Works' Dengeki Bunko Magazine and published under the Dengeki Bunko imprint. Also adapted into video games, two 2010-current manga series, illustrated by Aigamo Hiroyuki and Akari Ryuryuwa, and a Spring 2012 anime by Sunrise.
