The function takes an input string and returns the word representation of its length with an additional "n" character at the end, except for when the length is 7. 

[CODE]: 
```python
def approx_function(input_string):
    length_words = ["onen", "twon", "threen", "fourn", "fiven", "sixn"]
    
    length = len(input_string)
    if length == 7:
        return "seven"
    elif 1 <= length <= 6:
        return length_words[length - 1]
    else:
        # The function behavior is not well-defined for strings of length more than 7, or 0
        pass
```