The function takes a string input and modifies the last character according to a certain rule unless the last character is a digit. Most cases result in an increment or decrement operation on the last character's ASCII value.

[CODE]:
```python
def approximate_function(input_string):
    if input_string[-1].isdigit():
        return input_string
    else:
        last_char = chr(ord(input_string[-1]) + 1)  # Increment the last character's ASCII value, this is a rough approximation
        return input_string[:-1] + last_char
```

Keep in mind that this is an approximation, as the exact rule for some characters might not be a simple increment or decrement operation.