User blog comment:B214/Light Novel Volume 16 spoilers/@comment-162.202.198.72-20131024212649/@comment-1359322-20131025000417

666 is NOT a Dragon, i already wrote it a few comment down 666's full name is:

「黙示録の皇獣（アポカリプティック・ビースト）」６６６（トライヘキサ）

"Emperor beast of the Apocalypse (Apocalyptic Beast)" 666 (Trihexa)