riddle, secret, arcanum, paradox, conundrum, mystery story, mystery, closed book, whodunit, puzzler, brain-teaser, perplexity, show, parable.
He is an enigma,
a riddle that can probably be never solved."
The man was something of an enigma
and therefore roused in her an interest which was stronger because of some of the things she had heard to his discredit.
Ah, my good Colonel, give up the problem; it is an enigma
far above your powers to solve.