The truly unsettling part here is in the uncertainty of her existence.
She could be either dead or alive, despite hints pointing at the former. She could've gotten out after some more struggling, or somebody could've helped her. That, or the old one we see now is an apparition linked to the place of her supposed death.
Perhaps, she may have persisted in that form exactly because the boy never knew whether or not she escaped to live another day. Her immortality is fed by the very superposition maintained by his avoidance from observation.
Anyway, girl's probably a ghost. This is a cool interpretation of Schrodinger's Cat 🍜🐈