Help in strings please

Hi, I'm having problems with using strings on NIOS II IDE. Basically I plan to connect a RFID reader in my DE2 board, RFID sends 12 characters so I made codes to read them one by one and then store it in a string. Here's the C code for that part. I declared the "message" as a 12 characters. So I tried sending signals through Hyperterminal like 123456789012, which is 12 characters. When i print "message", it will print this instead "123456789012[][] a". The [] represents a box. So, what do you guys think I shall do?

unsigned char message;
 
  while(1)
  {
         // Read character from hyperterminal
        printf("Reading...\n");
        unsigned char k;
        for(k=0;k<12;k++)
            { 
        while((IORD_ALTERA_AVALON_UART_STATUS(UART_0_BASE) & 0x080) != 0x080) // Wait for read-ready
        {
        }
        
            message =  IORD_ALTERA_AVALON_UART_RXDATA(UART_0_BASE) ;     // Read character
            
            printf("Read: %c\n",message);
            }
            
            printf("Read Signal: %s\n",message);