It appears that characters are saved by their first name, so you can't use any of the names that are used by the pre-generated characters. However, these names show up when you click on the "generate a random name" button. So if you see "Anton" or "Maiwen" and think "hey, I like that, I'll use it", you'll be sadly out of luck because you won't be able to save the character with that name.
Simple "fix" is to remove the pre-generated character names from the list of available random names.
More complicated fix is to allow duplicate first names -- somehow detach the character file name from the character first name.
I guess another fix would be to allow us to delete the pre-generated characters.