A hero, whether on purpose or unwittingly, is someone that does what must be done for the greater good of those around them (without regard to their own situation or personal safety). Tom does this on several occasions. He has a deeply seated sense of justice about him, so when Becky is about to be whipped at school, he steps up and sets things right. He also saves Muff Potter by risking his own life by testifying in court about what he saw. He manages to save himself and Becky from McDougal's Cave.
