Saya (Saya no Uta)
She is INSIDE, so, no shoes.
The yellow ball she's holding is to represent the food she eats from Fuminori's POV.
Fuminori's medical procedure leaves his view of the world altered for the worst; blood and guts covered everything that seemed normal to others, and his friends appeared as horrible monsters. When all hope is lost for his sanity, he meets a girl named Saya. She is the only one that appears human in a world of heinous creatures.