This is Sheena, a Spirit of Reflections. She can only exist in reflections within a certain distance of her mirror, but never ever leave her side of the looking glass. A reflection spirit is born when two reflective surfaces (usually mirrors) are placed facing each other, creating the infinite hallway. A spirit (soul of a deceased person) will walk into that hallway, thinking it to be the tunnel to eternity, and become trapped once the mirrors are taken apart.
Sheena got lost in the Infinite Hallway so long ago, she has forgotten any time before her imprisonment. She still dreams of meadows and laughter, but always awakens cold and alone in her prison.