“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?