Genkaku Shoujo ga Tsukimatou

The protagonist is being followed around by a girl who appears to be just a hallucination. Note: The original web comic titled Genkaku Shoujo ga Tsukimatou Hanashi (幻覚少女がつきまとう話) that was released in 9 parts on Twitter and Pixiv has been compiled on Amazon into an e-book volume titled Genkaku Shoujo