Convert character to Integer while leaving encoded
OK, so after the lengthy requests for clarifications, it appears that
you are receiving a number in ASCII decimal (&quot;12&quot;) and want to parse
it into an integer.
You may use the parseInt() method of the serial object, like:
int c = SerialBT.parseInt();

This can however make you program quite slow, as parseInt() uses a
timeout to know when to stop expecting more digits.
If you already have read the bytes, you can combine them with simple
arithmetics:
int c = 10 * (a - '0') + (b - '0');

Note that '0' is the same as 48, only it makes more explicit the
intent of your code.