Big Sister Fenrir and I

Haijima Yousuke, bored of life, casually wishes for the world to be destroyed, even though he hates fighting and involving himself with people. And yet, when a terrifying monster suddenly appears and strikes down his beautiful upperclassman Kitami Touka right before him, Haijima knows that this time, he must fight. However, there seems to be more to Kitami than Haijima originally thought.