The function takes a string as input and increments the first letter's Unicode code point by 1, unless the first letter is a lowercase letter (a-z), an uppercase letter (A-Z), or a special character. In those cases, it keeps the first letter unchanged.

[CODE]:
```python
def function_approx(x: str) -> str:
    if x[0].islower() or x[0].isupper() or not x[0].isalpha():
        return x
    else:
        new_first_letter = chr(ord(x[0]) + 1)
        return new_first_letter + x[1:]
```
Please note that this code is an approximation of the behavior based on the inputs and outputs observed. There might be slight differences that we cannot confirm without the actual source code.