Why does Notepad++ display some Unicode characters as square?

It depends on whether the font you use has these symbols defined or not.

I found a basic way to change the font here:

• Choose the menu option Settings - Style Configurator…
• In the Style configurator dialog, choose the Global Styles language
• Then, select the Default Style style
• Finally, in the Font Style area, you can select your favourite font and/or its size, too

When I pick a font from the list, it is applied immediately, so it's easy to see which one supports characters you need. Of course if you use more than "Global Styles" / "Default Style" then you should modify the above solution accordingly.