Manga Like Yui wa Kage de Shika Ikirarenai

After school on a scorching summer day. Aki, a boy who's been bullied, resolves to confess his feelings to Yui, his childhood friend. But on the way home, Yui suddenly vanishes. They were definitely walking together. Aki, despairing at losing Yui, his only ally and the object of his affection, is overcome with grief. Then, Yui reappears before him. “I became a shadow.” Her form was strange. She extended limbs from within the shadows, moving as if swimming. Wherever shadows existed, she could freely enter and exit. The ground, the ceiling, even a person's body... Was she truly Yui... or something else? Aki scrambles to get Yui back. Yet, contrary to his wishes, the nightmare of the “shadow” begins to swallow them whole... (Source: GANMA!, translated)