@bg1987: baxslash have already given a hint.
@baxslash: Thanks.
@Quik: I hope you won't get disappointed!
@Bizar Guy: You can get the fallen key using an item, but you have to find it.
Sadly, I don't have enough papers to travel abroad. Maybe if the time comes, i'll try.
@Tahlkora: There's a key hanging somewhere. Try to always look up.
@TheComet: There are disadvantages with your suggestion:
-It has been used in movies already, so it's quite predictable.
-I will need to find a way to render it, either by duplicating the scene or by having another camera.
-Unfortunately, the girl's aim is not to get away from you. (I simply added that getting-away action for solving a minor technical issue)
Although I have another idea based from yours. You won't be able to see the girl from the mirror, but when you turn around, she's right there beside you.
But even so, I've decided not to touch the game anymore. It's done already anyway.