When looking at character description, you must look at action and dialogue. While he is being defiant towards his Aunt, are there specific reasons why? Has he said or done anything up to this point that would explain his actions of defiance? Sometimes we do things out of spite (to get back at someone), or we do something to get attention from someone. So, the fact that Tom is willing to do this to his Aunt, what does this tell us about the kind of person he is?