Without doubt, it is ...Brett Partridge.

Season 1, Jane calls him as "Ghoul".
Season 6, last scene on the wall "666", where Brett was about to die.

Ghoul Definition - One who delights in the revolting, morbid, or loathsome. 2. A grave robber. 3. An evil spirit or demon in....

666 Definition:

http://www.equip.org/bible_answers/what-is-the-meaning-of-666/

-- Multitudes today assume that 666 is a number representing a modern-day beast about to be revealed.

Jack Plotnick said, he didn't faked his death. Teresa call him as "Partridge".
Tyger Tyger, Symmetry give clues about twins.

Let's wait and see
Until 6x08, happy hunting....