Need a little help to fix an Arduino RFID program

I just extracted the problematic part of my program, I use RFID.h and SPI.h, I just want to know how to read on a RFID card (written with an android phone) I only write one letter : R, G, B, Y, ... (represent color) , on an Android tool I can See at sector 04 : ?TenR? When the "R" after Ten is the string that I wanna read :

    char buffer_data[8];
    rfid.read(0x04,buffer_data);
  
    String myString = String(buffer_data);
    Serial.println(myString);

I only want to know how to output => "R" (text on the RFID card at sector 04) : It output something like that :

22:05:15.885 -> 
22:05:15.885 -> &⸮
22:05:15.885 -> ⸮⸮

With other cards (Y, B char inside) same output...

Screenshot with card data (Mifare classic 1k (716B writable)):

The lib RFID.h with rfid.read doest not work... https://github.com/song940/RFID-RC522 don't use this lib !

The lib https://github.com/miguelbalboa/rfid is better, up to date, and can read most of tag types !

This is the fixed code to read the first text char on NTAG215 :

if (rfid.PICC_IsNewCardPresent()) {
 if ( ! rfid.PICC_ReadCardSerial()) {
   return;
 }
  Serial.println("");
  String str;
  byte buffer_data[18];
  byte size_data = sizeof(buffer_data);
  rfid.MIFARE_Read(4,buffer_data,&size_data);
  str=String((char *)buffer_data);
  Serial.println(str.charAt(9));
}

Ouput the first letter on the tag (if you write text data with Android NFC tools app ) only on NTAG215 (other tag = different adresses/position)!