It means that she is basically all he can think about. He will do anything or be anything for her, whether it be a brother, a friend, or a lover.
I was clapping for your awesome stories. SPOILER ALERT: I also read all the books...but I don't think the Volturi and the others fought because the Cullens brought other vampires with powers and the head of the Volturi didn't wanna lose those vampire with powers so he called it off...correct me I'm wrong :? The only people who had a "fight" was Bella and that other girl who could torture someone just by thinking of it/looking at them. It was more like a mental fight :?