éminence grise
Someone who is powerful in a particular area without having an official title. The phrase, French for "grey eminence," originally applied to one of Cardinal Richelieu's advisors, a monk who wore a grey robe. A: "Why should I worry about what Bob thinks? He doesn't even have a title!" B: "Yes, but he's the éminence grise of the firm, and the partners really value his opinion."
gray eminence
The power behind the throne; a person who wields considerable power but secretly or surreptitiously. The term is a translation from the French of éminence grise. This phrase originally referred to François Leclerc du Tremblay (1577–1638), the trusted behind-the-scenes adviser of Cardinal Richelieu. The term came into English in the early 1940s when Aldous Huxley wrote a book about Tremblay entitled Grey Eminence (1941). See also power behind the throne.