Getting gibberish in serial terminal (Bluetooth)

Development Boards Arduino
2

That is a very common problem when connecting new serial devices.

First try different Baud rates.

I see your RealTerm is set to 19.2k Baud and you code is set to:

Serial.begin(115200);

Next ensure that your device is sending ASCII characters.

  Serial.println(counter);
  counter++;

This would be sending BINARY.