Just then, the Aurora cast off from the Eiffel Tower and was free again, gracefully rising and turning in the wind to begin a new journey. "That's my girl," I said, and took her hand. (21.117)
The last paragraph of the book has some really sweet ambiguity, doesn't it? Matt and Kate have just had a nice little reunion over tea, and they are discussing the potential to be near each other for the next few years while they both pursue their dreams. But when Matt says, "That's my girl" is he referring to Kate, or the Aurora?
Clearly, as he takes Kate's hand, there is an attraction there, and they've basically just done the stereotypical let's-go-to-college-near-each-other-so-our-love-can-last plan that most teenagers make if they have a high school sweetheart. But ships are commonly referred to as females, and we all know how much Matt loves his Aurora… So which do you think he's calling his girl? Maybe we'll find out in the sequel.