gunman, assassin, sniper,
Hidden, covered, censored, blocked
sheltered, private, concealed, hidden, unfrequented, sequestered
If you mean letters, it's hidden.
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.
secretive, mysterious, concealed, covered,
He concealed the message in the secret location. The sniper concealed himself carefully beside the tree.
hidden cryptic concealed
covered