she is a half ghost and studying in same school with those three, but she just realize/or forget that almost everyone in the school are not human<including herself>?
Somebody missed the Keine/Ex-Keine tag. That's her in the first panel for the Test of Courage strip (she's in Ex-form so that's why the students don't recognize her and her horn has the ribbon).
Waaaaaagh!Test of CourageWithout EqualWhoo-hoo!I'm scared of ghosts...We will now begin the test of courage!Quit playing around, all of you, and get to bed now!Who's she...?Pillow FightHeyyy!They're not humaaaaan!Ooohhh...There's stuff growing out of 'em!