The Brave and The Demon King

There exists a country that seeks the destruction of mankind, "The Devil's Castle Federation". In order to defeat the strongest demon that reigns there, the "Demon Lord", many Heroes stepped up, only to be defeated, never to be seen again. And now a new Hero is rising up again!! But that hero is a…… little girl!?
The legendary battle for the survival of mankind begins…… Uh, maybe?