esoteric

erudite, learned, scholarly

academic (also academical), pedantic

complex, complicated, hard

darkling, enigmatic (also enigmatical), inscrutable, mysterious, mystic, mystical, uncanny

impenetrable, incomprehensible, unfathomable, unintelligible

ambiguous, cryptic

unanswerable, unknowable

baffling, bewildering, confounding, confusing, disorienting, mystifying, perplexing, puzzling

classified, restricted, top secret

silent, unadvertised, unannounced, undisclosed, unmentioned, unsaid, untold

clandestine, closet, collusive, conspiratorial, covert

furtive, hugger-mugger, occult, sneak, sneaking, sneaky, stealthy, surreptitious, undercover, underground, underhand, underhanded

personal

closeted, concealed, hidden

repressed, silenced, stifled, suppressed

backstage, offscreen, offstage