IDIOMS

éminence grise

(idiomatic) A secret or unofficial decision-maker; the power behind the throne.

Example: 1987 July 12, John A. Garraty, "Harry's Luck" (book review of Harry Hopkins: Ally of the Poor and Defender of Democracy by George McJimsey), New York Times (retrieved 4 Sep. 2011):
  But Harry Hopkins was no mere manipulator of people and ideas, no eminence grise masterminding the actions of figureheads.
2009 Sept. 16, Coco Masters, "New Prime Minister — and New Shadow Shogun," Time:
  Ozawa is the Secretary-General of Hatoyama's Democratic Party of Japan (DJP) and the éminence grise of the electoral campaign that catapulted the new Prime Minister to power.