Symbolism, Imagery, Allegory
The image of a butterfly comes up again and again in Luna. It's really fitting because Luna desperately wants to be like a butterfly: she wants to emerge as a beautiful girl and reveal herself to the world. She is captivated by the idea of transformation:
The other Teri Lynn, the real one, had blossomed and sprung to life. The way Liam broke free when he morphed into Luna.
When Luna sees Teri Lynn's transformation, she is inspired—she sees that with some changes, her life could be completely different. Right now though, she has to live as a butterfly who is still cocooned. As Regan puts it:
Like a butterfly emerging from a chrysalis, I thought. An exquisite and delicate creature, unfolding her wings and flying away. Except in Luna's case, the butterfly is forced to rein in her wings and reinsert herself into the cocoon ever day. Every single day, she has to become this shell of a person. (14.98)
It is only when Luna is brave enough to completely let go of her safe daytime cocoon—a.k.a. Liam—that she actually takes control of her life. When she emerges as Luna at the end of the book, Regan knows that she is never going back into the cocoon of Liam. She is never going to be her brother anymore. Luna's realized her butterfly ideal—that of a brave young woman.