Gen is an incarnation of demon. Three thousand years ago, a war happened between the god and the demons. Gen was known as General X, leading the demons to rebel against the god as commanded by Lucifer.
Gen-X had successfully penetrated god's fortress, the victory was supposed to lie on his side but he somehow cast a spell and caused a big explosion. It was a self-destruct spell to conceal his power and his soul. No one knows the reason Gen-X did it.
During the great explosion, some of the gods and demons were transported to the human world. Both then secretly organized a group. The legend was passed down since then in search of the concealed power. It was believed that the soul of Gen-X would revive after 2 millennia...