soul of, the
The essence of some quality, as in You can trust her; she's the soul of discretion, or He's the very soul of generosity but he can be cranky at times. This idiom was first recorded in 1605.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.