from the answer to a very similar question from 2014:
- Log in into the Mac as administrator.
- Backup the Mac with Time Machine (optional but recommended)
- Launch System Preferences selecting System Preferences from the Apple menu.
- Click the Security & Privacy preference pane.
- Click the FileVault tab.
- Unlock the pane clicking on the padlock (administrator password required)
- Disable FileVault and wait (…about 3hrs with a 250GB no SSD HD)
- Restart the Mac (optional but recommended)
- Go back the FileVault tab.
- Enable FileVault and wait (…about 7hrs for me!!!)
- Restart the Mac.
- The unknown user account will disappear.
- This should also now allow you to set automatic login.
I assume, if thats the only account listed, it has to be an administrator account.
I know you might not be able or want to back up your drive again, though. So maybe this isnt what youre looking for (though that step is optional).
also heres a discussion on apple.com from 2011 with a similar question and answer. this one may be closer to your scenario as it also doesnt mention any accounts besides the "update needed" account