Do not forget that the protagonist in a story is the main or principle character and the antagonist is that character's enemy or the character who presents opposition to the protagonist.
Because this is a short play with a small cast, there are a couple of characters who would best be considered protagonists.
The core group includes Stephen "Red" Ryder, from the title of the play, Angel, the overweight waitress who has a crush on Stephen, and Lyle, the crippled owner of the gas station and motel. It could be argued that all of the characters in the diner that day have nearly equal roles, but because these three characters are fixtures in the town where the play is set, I would consider them to be the best choices as protagonists.
The antagonist of the play is of course Teddy, who stops at the diner because his van needs a generator. When it becomes known that he is smuggling drugs, the subtle tension he created upon entry escalates to the climax of the play. As an antagonist, Teddy is power hungry, manipulative, violent, and unpredictable. He is a classic villain in a classic story of the disruption of life for normal people in a normal town.