She is the voice of reason, the dark pillar of mystique that knows his faults to a 'T'. She may not understand completely, but perhaps it is better that way. Her solitude allows her to be just, and her acquaintance with the darkness keeps her aware of the fine line between it and that other place.
That place she so desperately pushes him towards with all her might.
He wonders a bit, why that is. Why is she so adamant about him doing the right thing? She's a sheltered little rich girl with a mind wrapped around the thought of despair. How is it that she is so aware of the difference between right and wrong, when he, the hero, is not? Shouldn't it be the other way around?
Maybe that's why they are so close. They've fallen into each other's places.
He does not love her in the way that most people assume, (his heart, in fact, belongs to another) but he does love her. She is his sister and his mother and guardian.
But most importantly, she is his best friend.