This is the second anime adaptation of the manga series. The first is a yet to be completed, OVA series from 2012.
Based on a Japanese manga series written and illustrated by Hiroki Adachi, under the pseudonym Hero. It was self-published as a webcomic on Hero's website, Dokkai Ahen, from February 2007 to December 2011 in a four-panel format. The series received a print publication by Square Enix, who compiled the chapters in ten tankobon volumes under its imprint Gangan Comics from October 2008 to December 2011.