The function replaces every other character (starting with the second character) in a string with the character preceding it, except for special characters and numbers.

[CODE]:
```
def approx_function(s):
    result = ""
    for i in range(len(s)):
        if i % 2 == 1 and s[i].isalpha():
            result += s[i - 1]
        else:
            result += s[i]
    return result
```

Please let me know if there's anything else I should investigate or if you need more inputs/output pairs.