The function takes a single string input and decrements the last alphabetical character by one. It leaves special characters and numbers unchanged.

[CODE]:
```
def approximate_function(s: str) -> str:
    if s[-1].isalpha():
        last_char = chr(ord(s[-1]) - 1) if s[-1].lower() != 'a' else chr(ord(s[-1]) + 25)
        return s[:-1] + last_char
    return s
```