Defaultuser0 is a temporary account created by the system itself. When installing Windows 10, the Windows OOBE (Out-of-box experience) can’t run without an existing user account. Therefore, if there is no other created account, defaultuser0 is created to start OOBE.
Defaultuser0 doesn’t belong to any user and doesn’t have any configuration files. When it appears on the login screen, it requires the user to enter a password.