Memory

Oops they're dead. They were just a happy Mew before all this, but when they died, everyone forgot about them immediately.

This caused them to become extremely upset. So now they're just an angry spirit who tries to mess with everyone else for "revenge".

They pretty much just make people think about their good memories in attempt of making them upset about how those things are gone.

Nobody can see their face. It's constantly covered by white glitchy stuff. The rest of them can often be seen fading in and out of existence too.