A 2008-2018 light novel series by Ishibumi Ichiei, with illustrations by Miyama-Zero, serialized in Fujimi Shobo's Dragon Magazine. Also adapted into a 2011-2018 manga, with illustrations by Mishima Hiroji, serialized in Fujimi Shobo's Monthly Dragon Age, and an anime by TNK for Seasons 1 - 3 (2012 - 2015), and Passione for the fourth season (2018).
