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Defaultuser0 is a system account Windows 10 is using in setup phase. You can see this yourself next time you will do a clean install: when Windows installation after last restart arrives to account creation screen, press SHIFT + F10 to open a Command Prompt and jump to current user's profile folder with command cd %userprofile%:

(Click to enlarge.)
You can see that the above command takes you to defaultuser0 folder, meaning that you are signed in to Windows using this system user profile until you create your initial user account and sign in with it instead. As the profile / account will not be needed after Windows and its initial admin user profile have been fully set up, it will disappear / be removed automatically after a few restarts in a day or two. If it bothers you it is totally safe to delete defaultuser0 profile folder manually.
No worries, no issues.
The hidden user profile Default is the base profile used for all new user profiles. When you create a new user account its desktop, Start and other settings are copied from Default user profile. It is a very important system profile folder and should not be removed. Keep it hidden.
Kari