The function counts the number of 'a' characters in the input string and returns the word representation of that count.

[CODE]:
```python
def count_a_chars(input_str):
    num_a = input_str.count('a')
    num_to_word = {
        0: 'zero',
        1: 'one',
        2: 'two',
        3: 'three',
        4: 'four',
        5: 'five',
        6: 'six',
        7: 'seven',
        8: 'eight',
        9: 'nine'
        # Add more if necessary
    }
    return num_to_word[num_a]
```
Feel free to test this code implementation for desired inputs and let me know if you need any more information or assistance!