User blog comment:B214/Light Novel Volume 16 spoilers/@comment-162.202.198.72-20131024212649

I am guessing that 666 is like the equivalent of Acnalogia from Fairy Tail in Dxd because he probably represents the counterpart of Great Red like for example" True Dragon God of Apocalypse, and Destruction", which means he represents death and ultimate destruction for everyone but this is just my guess.