“So what part of it isn't living up to your expectations?" asked Harry. Anger was coming to his defence now. "Did you think we'd be staying in five star hotels? Finding a Horcrux every other day? Did you think you'd be back to Mummy be Christmas?"
Harry Potter and the Deathly Hallows
I understand the phrase "come to one's defence" as in His friends quickly came to his defense. The subject should be a person. But in this case, it's 'anger'. How should we understand it correctly?
What this means is that Harry was getting defensive and slightly angry (at that particular moment). When we have built up emotions inside and someone attacks us (complains, blames, criticisms, etc.), we tend to get defensive. And it is quite natural to get angry when we are defensive.
Ron was sort of complaining and blaming Harry ("We thought you knew what you were doing!” shouted Ron, ... we thought you had a real plan!").
I am guessing this is used as figurative language - doing so brings emphasis on "anger" as if it were a being.
From the context, it seems that Harry was not fully angry until after he was taken out of the tent.
