I've been traveling, and it's been hard to post,so I'm glad the discussion is still open.
Marcie found the fact that the author thinks of the book as a trio sonata. To me, it's a slightly different musical form--a rondo. In a rondo, the main theme is played, then a different section, then back to the main theme, a second different stretch, back to the main theme, alternating back and forth several more times, finally ending with the main theme.
Arrow is the main theme here. She shows the most intense, focussed, and pared-down version of the struggle all the characters are having, and we start and end with her. She does manage to pull back from dehumanization and keep her own identity. It will cost her her life, but she thinkt it's a fair trade.