A protagonist is the central character or characters in a story who is always involved in the main conflict. The character(s) is usually one with whom the reader can identify. I would say the protagonists in this play are Karen Wright and Martha Dobie. The antagonist, the
character(s) who is opposition to the protagonists, has to be Mary Tilford.
Karen and Martha are victims of Mary's lies and vindictive behavior. I think all of us can identify with the hurt that occurs with being the victim of gossip or lies. Mary is spoiled and clever, and Martha and Karen see through her behavior. As a result, they become the victims of her malicious slander that results in Martha's suicide.