How about "The soul of discretion"?
Promise you won't tell anyone?
I promise! I'm the soul of discretion.
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Tight lipped.
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He likes to keep to himself
He's not much of a talker
He's an introvert
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Thank you! :)
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!
A true friend. A Sphinx. A confidante. Maybe a clam.
Often you'd recast this to use an adjective like close-mouthed, tight-lipped, reliable, discreet.