The protagonist of the short story "Sredni Vashtar" is Conradin, a ten year-old sickly child who lives with his cousin, Mrs. de Ropp. She is his antagonist.

The protagonist of the story is the main character. On the other hand, the antagonist is the adversary or opponent that prevents the main character from achieving a goal. An antagonist could be a person, nature, the environment, or circumstances. Therefore, in the case of Mrs. de Ropp, she is an antagonist because her dislike for Conradin makes her do things that limit him and make him very unhappy. 

Mrs. de Ropp was the ground plan on which he based and detested all respectability

Mrs. de Ropp did many things to upset Conradin. She acted sanctimoniously toward him, made him go to church services, made him feel that even making him a piece of toast for breakfast was troublesome, and was quite supportive of the medical doctor’s opinion that Conradin would not live for a very long time; that he would be dead within the next five years. To this, Conradin felt a suppressed anger that later on became channeled by his made-up worship of a ferret  whom he names Sredni Vashtar. The ultimate wish of Conradin is for the animal to do a miracle for him. His faith in it is quite extreme. When the ferret attacks and kills Mrs. de Ropp, Conradin believes that his miracle has been granted.

In essence, in the conflict between the protagonist Conradin and the antagonist Mrs. de Ropp  the protagonist was victorious.