"frank" suggests "willing to reveal"
"secretive" suggests "unwilling to reveal"
These two are adjectives.
The corresponding nouns are "frankness" and "secretiveness", "the
quality or property of being frank", "the quality or property of being
secretive".
(A noun cannot be the antonym of an adjective, so the relationship
'frank: secretiveness' is not simply that of opposites.)
callow: maturity seems to be the only similar combination.
CJ