The function takes a string input and replaces each character at an odd index with its next character in the alphabet (if the startIndex is considered 0). For example, every second letter is replaced with the next letter (i.e., index 1, 3, 5, ...), and all other characters remain unchanged.

[CODE]: 
```python
def approximate_function(input_string):
    output_string = ""
    for i, char in enumerate(input_string):
        if i % 2 == 1 and char.isalpha():
            next_char = chr(ord(char) + 1)
            output_string += next_char
        else:
            output_string += char
    return output_string
```