power behind the throne
A person with great influence who stays behind the scenes and has no apparent authority. For example, Harry may be the CEO, but it's obvious that his wife is the power behind the throne. [Mid-1800s]
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.