This calculated field returns the user who wrote the most recent comment in a text field.
Create a new Calculated Text Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional.
Select Advanced text as parsing mode.
Add the following expression:
Add the field to the relevant view screens.
Remember: All calculated fields are read-only and cannot be manually updated on a create or edit screen.
Check the results
Once configured and added to an issue screen, the result will look like this: