Montag is directly characterized as an unhappy person. An example of this is that when Clarisse asks him if he is happy he has to convince himself that he is happy. We are given clues that he is unhappy at his job since he is in charge of burning books, but he has books himself. Montag is also unhappy with his wife and all the sleeping pills she takes. This characterization is direct since he asks himself if he is happy.
Montag is indirectly characterized as a rebel. This is shown through his harboring of books. During a fire Montag takes a book from the pile to be burned. The author doesn't directly say that he is a rebel but it is shown through his actions.