An antagonist is the person standing in the way of the protagonist. Our protagonist, Boy Willie, is determined to sell the piano, and his sister, Berniece, is determined to stop him. This makes her – you guessed it – the antagonist. Sometimes people think of antagonists as "villains," and, of course, sometimes they are. Berniece doesn't really fit this description, though. Chances are, some audience members even sympathize with her more than her brother.
At the end of the play, a new antagonist rears his ugly head – Sutter's ghost. Now, this creature is a straight-up villain. Not only is he the spirit of the family that once owned Berniece and Boy Willie's, but he also was most likely responsible for the death of their father. Now he shows up and starts throwing Boy Willie down the stairs. Sounds like an antagonist to me. The ghost is such a bad guy that Berniece and Boy Willie put aside their differences and unite to fight him. The fact that Berniece has been a really sympathetic antagonist the whole time works in the play's favor when she assumes the protagonist role at the end of the play to banish Sutter's ghost.