The function f(x) takes a string as input and replaces any occurrence of the letter 'e' with 'u'. All other characters remain unchanged.

[CODE]:
```python
def f(x):
    return x.replace('e', 'u')
```

This code snippet demonstrates a simple implementation of the observed behavior. The function uses the `replace()` method in Python to replace all occurrences of 'e' with 'u' in the input string.