Gen is an incarnation of demon. Three thousand years ago, a war happened between the God and the Demon. Gen was known General X, leading the Demon to rebel against the God as commanded by Lucifer.
GenX had successfully penetrated God's fortress, the victory were supposed to lie on his side but he somehow casted 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 GenX 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 GenX would revive after 2 millenium...