Luna is an anthropomorphic Doe, and a symbol for motherly and sisterly love to Andrew.
She serves as a guide throught the series. But even when she is unsure of where to lead him, she helps Andrew continue on his journey with constant support as a friend and elder. He has the most love and affections for her, even before Lily.
Luna takes the form of a feral doe whenever she is facing the task of directing Andrew and his companions, but takes a more human form in any personal situation, good or bad.
She is most prominent face to face in the land of the Terra Nubelesus, giving Andrew direction and support as he encounters and gains the trust of the diverse, colossal Gods.