Blackrat
He Who Lurks Beyond The Veil
Or maybe that person was brainwashed by another Illuminati group to believe he's a member of the Illuminati in an attempt to discredit their Illuminati rivals.
Could be... They are sneaky lot.
I wouldn't even want to be a member, but that's 'cause I belong to an even greater secret society. The Brotherhood of Franks... It's so secret that some of the members don't even know themselves that they belong to it.
Hmm... Maybe it's time to start spreading the Brotherhood abroad...