Miss Strangeworth is the main character in Jackson's story.
"The Possibility of Evil" takes place in an unnamed, small town. While the town could be anywhere, there can only be one main character. The story's primary focus is Adela Strangeworth. She "knows everyone in town" and had pretty much lived in the town all her life. Essentially, she is the means through which we know the townspeople. She is a "busy body," someone who finds it important to know information that would prevent the presence of evil in the town that she loves so much.
Jackson explores the story's themes through Miss Strangeworth. The themes of duplicity amongst people, the right to be left alone, and how good intentions can lead to disastrous consequences are all a part of Miss Strangeworth's characterization. She is the reason why so many people in the town are acting in a different way. Jackson writes that "Many people seemed disturbed recently" because of Miss Strangeworth's letters. Miss Strangeworth initiates the story's plot. She is the main character because the entirety of the text is about her and her interactions with others. While the other characters in the story are interchangeable, Miss Strangeworth occupies great importance and serves as the protagonist in Jackson's short story.