The function performs a ROT13 cipher on the input strings, shifting each letter by 13 positions in the alphabet. Numbers and special characters are not changed.

[CODE]:
```python
def rot13_function(input_string):
    result = []
    
    for char in input_string:
        if 'A' <= char <= 'Z':  # Uppercase letters
            result.append(chr(((ord(char) - ord('A') + 13) % 26) + ord('A')))
        elif 'a' <= char <= 'z':  # Lowercase letters
            result.append(chr(((ord(char) - ord('a') + 13) % 26) + ord('a')))
        else: # Numbers or special characters
            result.append(char)
            
    return ''.join(result)
```