Who is the antagonist in the novel, Tales of a Fourth Grade Nothing?

Sheila Tubman, the novel's antagonist, is something of a know-it-all who tries to seem important in most situations. She often teases Jimmy and Peter about “cooties” and she tries to be in charge when the three are teamed up to work on a school project. She convinces Mrs. Hatcher to leave Fudge in the park with her one day, insisting that she is a good babysitter.