Randomize String Array and display all of the values

Getting a blank button or just with "Masons" or "Villager" String

That is because you only set the Button's text with the last element of the list. Which is either null or "Masons" (not seeing how it could be "Villager").

for (int s = 1; s < characters.length; s++)
{
    randomButton.setText(characters[s]);
}

If I check the log I see that all the others cards(characters) get displayed as "null"

You only set position 0 of your array. For example, you don't initialize the positions, so these int values default to 0.

int demoniac;
int guard;
int all;

Then

for(i = 0; i < all; i++){
    add(demoniac, "Demoniac");
    add(guard, "Guard");

Really, that loop shouldn't be entered because all equals 0.

Additionally

Collections are zero-indexed, so this doesn't print element 0. You need to set int s = 0;.

for (int s = 1; s < characters.length; s++)

It isn't clear to me what the add(int character, String name) method is returning, but if you explain it, I will update this answer.

I believe this code fulfills most of what you are trying to achieve

// Where the characters are stored
private ArrayList<String> characters;

public void initDeck() {
    if (characters == null)
        characters = new ArrayList<String>();
    // Extract the numbers if you actually need them, otherwise, they just are constants
    addCharacter("Demoniac", 1, characters);
    addCharacter("Guard", 1, characters);
    addCharacter("Medium", 1, characters);
    addCharacter("Mythomaniac", 1, characters);
    addCharacter("Owl", 1, characters);
    addCharacter("Werehamster", 1, characters);
    addCharacter("Villager", 5, characters);
    addCharacter("Masons", 1, characters);
}

public void addCharacter(String name, int amount, ArrayList<String> cards) {
    if (amount < 0) {
        throw new IllegalArgumentException("Must add a non-negative number of characters for " + name);
    }

    // Don't use '==' for Strings
    if (name.equals("Villager")) {
        if (amount != 5 || amount != 12) {
            throw new IllegalArgumentException("There can only be 5 or 12 " + name);
        }
    }

    for (int i = 0; i < amount; i++) {
        cards.add(name);
    }
}

public int searchCharacters(String character, ArrayList<String> cards) {
    return cards.indexOf(character);
}

public Map<String, Integer> getAllCharacterPositions() {
    Map<String, Integer> allPositions = new LinkedHashMap<String, Integer>();
    for (int i = 0; i < characters.size(); i++) {
        allPositions.put(characters.get(i), i);
    }
    return allPositions;
}

void run() {
    // initialize the characters
    initDeck();

    // shuffle them
    Collections.shuffle(characters);

    // print them all out
    for (int i = 0; i < characters.size(); i++) {
        System.out.printf("%d: %s\n", i, characters.get(i));
    }

    // Find the position of a character
    System.out.println();
    String findCharacter = "Owl";
    // Option 1 -- always linear search lookup
    System.out.printf("%d: %s\n", searchCharacters(findCharacter, characters), findCharacter);
    // Option 2 -- one-time linear scan, constant lookup
    Map<String, Integer> positions = getAllCharacterPositions();
    System.out.printf("%d: %s\n", positions.get(findCharacter), findCharacter);

    // Get a random character
    System.out.println();
    Random rand = new Random(System.currentTimeMillis());
    int randPos = rand.nextInt(characters.size());
    System.out.printf("%d: %s\n", randPos, characters.get(randPos));

    // randomButton.setText(characters.get(randPos));
}