The function takes an input string, performs an operation on the first character, and inserts '5' after the first character.

[CODE]: 
```python
def modified_function(input_string):
    first_char = input_string[0]
    # Operation on the first character
    new_first_char = chr(ord(first_char) + 2)
    return new_first_char + '5' + input_string[1:]
```
Note that this code might not correctly replicate the operation on the first character, but it appears to be close based on the input-output pairs provided. Please test and confirm this code's functionality with some inputs.