a person who exercises power indirectly through his or her often unsuspected influence on somebody with another person or group of people who have titular authority
French éminence grise, literally ‘grey eminence’, nickname of Père Joseph (François du Tremblay) d.1638, French monk and diplomat. He was the confidant of Cardinal Richelieu d.1642, French statesman, who was known as Éminence Rouge red eminence. The names are from the colours of their respective habits