gunman, assassin, sniper,
Hidden, covered, censored, blocked
If you mean letters, it's hidden.
sheltered, private, concealed, hidden, unfrequented, sequestered
Another word for conceal is to hide, disguise and hold back.
pistol, firearm, shooter, handgun
Another word would be drenched, coated or smothered.
He had already concealed the box
An adverb is a descriptive word that modifies a verb. 'Concealed' does not modify a verb. Concealed is an action word, a verb, the past tense form of conceal.
She concealed her true feelings behind a smile during the meeting.
secretive, mysterious, concealed, covered,
hidden cryptic concealed
concealed, obscure.