Studio Key Works Kanon - Mai's... *potential spoilers*

Well, for those of us who have seen Kanon, we're well aware of Mai's story arc. She stalks the halls late at night, slaying invisible demons.

Now, my question is, are these demons mere figments of her imagination, or are they real beings? Or better, where could they have come from?

If they're the former, for example, then how is it that Yuuichi can see them also, and how is it that Sayuri was so badly injured by one of them?

Love to hear your guys' thoughts on this.