Demonizer Zilch Vol. 1 (manga)

Haunted by the untimely deaths of his mother, little sister, and a childhood friend, listless high schooler Haruomi Io has trouble engaging with anything or anyone. Of course, that doesn't mean others have trouble engaging with him-a fact he discovers to his dismay when he finds himself the target of a trigger-happy group calling themselves the "Seventh Gospel Organization." Having newfound enemies may be rough, but Haruomi's unexpected ally in the form of a demoness calling herself Zilch may turn out to be even more of a handful...Still, there's something to be said for a dose of the supernatural (and more than a few buxom girls) when it comes to casting off the doldrums!