No special reason,anything can be used.
Sure. But, some names, like cell make sense. Other names, like d8RhgWLgpzx, don't. While the program you are developing is called code, it doesn't have to be cryptic.
mySerial.read() returns int type
With a error flag in the high order byte and the data in the low order byte. If you only call Serial.read() after calling Serial.available() to confirm that there is serial data to read, you can store the output in a byte-sized variable, like byte, char, or uint8_t.
and i got series of special character(box like)when tried to display it using serial monitor.
Looking at your code, I can't see why. Of course, the fact that I can't see your code might have something to do with that.