The soul of discretion noun phrase
Someone who keeps their lips shut and can be trusted with confidential information.
Tom is a reliable guy. You can trust him to be the soul of discretion.
I don't know about that girl. She doesn't seem like the soul of discretion.
This phrase was found in Phyllis of Philistia, an 1895 romantic novel by Frank Frankfort Moore:
“ ‘You are the soul of discretion, my beloved,’ said the husband.”