The Five Worlds

A dwarf runs to you. He needs your answer for his son's homework. If you are correct, he will provide you food and bed. If you are wrong, he will train your blood and split your body into 3parts. Here is the riddle:
I am a human being.
I am alive.
I stand in the middle of the hallway,
But no one notices me.
I say hi to a girl,
But I receive no response.
I run in the hallway,
but no teacher stops me.
I look at everyone.
I am transparent and belong to nowhere.

What is the answer?