Character Role Analysis
In a sense, everyone's an antagonist. The characters consider Gregor so alien to anything, well, human, that they have a hard time establishing any sort of relationship with him. But Mr. Samsa seems to be the primary antagonist in Gregor's life. It is Mr. Samsa's position as head of family that Gregor usurps when the family business collapses, and Mr. Samsa is restored to his position of authority in the house after Gregor's transformation.
(Psst: Kafka had some pretty intense daddy issues.)