After analyzing the input-output pairs, I have come up with the following description and Python code that approximates the behavior of the function.

[DESCRIPTION]:
The function changes the last character of the input string based on the following rules:
1. If the last character is a lowercase letter (a-z), it will be replaced by the character whose ASCII value is equal to the difference between the ASCII values of 'z' and the last character of the input string, plus the ASCII value of 'a'.
2. If the last character is not a lowercase letter, the string remains unchanged.

[CODE]:
```
def approximate_function(s):
    if s[-1].islower():
        new_char = chr((ord('z') - ord(s[-1])) + ord('a'))
        return s[:-1] + new_char
    else:
        return s
```

Please note that this is an approximation of the original function and might not be an exact match.