ONCE UPON A GLASHMA (Hajimari no Glashma), by Kumiko Suekane. First published in 2006 and first published in North America in 2007.
PLOT:
One day, the women of the world disappeared and a few select men began to develop magical powers. It's the duty of the Ministry of Internal Affairs and Communication to take a census of the known wizards of the world. Thus, it's up to investigators like Nippori and his eyepatch-wearing, bat-wielding Sempai to go out and confront unregistered wizards and keep their powers under control. During the course of their work, Nippori discovers that Sempai knows more than he lets on about what caused all the women to disappear, and it's a secret that threatens to destroy what's left of the world...
