Who is this guy?

The Drink!

Tell me about him or her...

It's a him.  He switches cups with himself.  He looks like a cup.  Then when someone picks him up they scream because they see his face.  He says to other people that know that he is alive and are not scared of him that he is looking for someone, but he is actually not.  

Why does he do this?

None of your business bozo!