User blog comment:B214/Shin DxD 1 discussion/@comment-72.65.52.236-20180801180036/@comment-31137559-20180802025701

And Mephisto Pheles is an Extra Demon who is of the same generation as the original Maou. Also the devil who formed a pact with Georg Faust, who is the ancestor of Georg of the Hero Faction. After Georg Faust died, he took over as the leader of Grauzauberer. The organization is closely affiliated with the devils and fallen angels.

He is also Tannin's king using a queen piece to reincarnate him as a devil.

And Lavinia Reni is his subordinate as she's a member of the organization.