It's easy to lump antagonists in the category of "bad guys." In this case, we think it's more accurate to say that Julian's mother is an "agent" of antagonism. What we're trying to say is, Julian's mother gets in the way of Julian's happiness and goals because of her way of being and views on the world, which directly oppose his own.
Or, at least that's what he thinks. We're kind of wondering if Julian isn't his own worst enemy.