The best ratio of answers to questions is 1. Higher than 1 is much better than lower than 1.
Now, we know that you are using the Serial Monitor application, but not what options you have set, what you are sending or when.
My purpose is to send ASCII codes (so composed by one ore more digits) to Arduino and get back the corresponding character....is it possible...?
Yes, it is. It is quite easy, in fact.
There are two ways to go about it. One is to always send three characters, and hope like hell none get lost (not a realistic thing to do). The other is to send the value followed by a delimiter. The Serial Monitor makes this option easy.
Then, you read the data on the Arduino, storing it in a NULL terminated char array until the delimiter arrives. When that happens, you convert the array contents to an int, using atoi(), send a response, and reset the array index to 0 and put a NULL in that position of the array.
Though why you want to do this, versus looking in an ASCII table, is unclear.