The function takes an input string of a single character and converts it to its corresponding number as a word (e.g., "a" or "A" becomes "one", "b" or "B" becomes "two", etc.) up to the ninth letter. Any non-letter character is transformed to "zero".

[CODE]:
```python
def f(x):
    words = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"]
    if x.isalpha() and x.lower() <= "i":
        return words[ord(x.lower()) - ord('a') + 1]
    else:
        return words[0]
```