Nephilim are giant humanoids who terrorized the early inhabitants of Mecha Iwaku. They were eventually tamed, leading to the construction of a great society on their labor, led by the "angels", nobles who controlled these great beasts.
The Nephilim mysteriously died out, leading to the collapse of ancient Iwakuan civilization and the warring states period. Recently they have returned in the form of the N-00-Bs, extraterrestrial raiders who seem to be an alien version of the Iwakuan Nephilim. The Angelic Restoration Cult seeks a return to the Age of Nephilim, and Nephilim serve as the inspiration for their mecha.