(Spoilers) Summary for any confusion after reading: Mei, the girl, summoned a ghost which she thought was her father. (She thought he might've died already, from her mom's sarcasim) However, another 'father' ghost appeared. When Mei asked him to be with her, he asked her not to turn around (Since she'll find out he's not her actual dad) He's been with her since then until she's married. I think Mei realised along the line that he's not her biological father but since he had always been there for Mei, he's her papa. He didn't become human when he walked Mei down the isle. We can still see her hand through his arms. Mei was the only one who could see him. In the end, Papa moved on after full filling his dream of walking his daughter down the isle. That's why Mei cried, because Papa walked her down the isle and he vanishes before her eyes, gone forever. The reason Papa loves Mei so much was because before he died, he had a daughter around the same age as Mei (Protagonist)
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The Story of My Papa Who is Always Behind Me