Yoshimi WAS Mitsuko's mother

For one, Yoshimi moved to a new apartment, she never lived there before. Or at least it's not indicated in the movie (all we see is that she tells the mediator, she will soon chose an apartment, or something like that). And also nothing indicates that she had a daughter before Ikuko. Let's say she had: why would she abandon her?
My understanding is that Yoshimi realizes that Mitsuko's spirit would never leave them alone, because she wants a mother, so Yoshimi pretends to be her mother in order to save her actual daughter (Mitsuko already tried to kill Ikuko once).

That's an interesting hypothesis. Yoshimi did admit to having mental problems and fuguing out when she was younger. It's entirely possible that she blocked out a goodly portion of her life, including an elder daughter.

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"House. My room. Can't walk. My medal. My father. Father, don't!"

haha, you almost got me here. the aunt scene refutes your theory, she praised her as being better than her mother. the aunt wouldn't praise her if she abandoned a child before, she would know.

The man who works in the kindergraten talks to Yoshimi about the missing girl and I think it's clear he knew her and the family so I think he would recognise Yoshimi if she was the missing girl's mother. But there is no suggestion from him that there is any connection between Yoshimi and the missing girl.

The same goes for the people running the apartment block, they knew the family and would surely know if it was the same woman, but there is again no indication that this is the case

I agree that it's quite possible Yoshimi WAS Mitsuko's mother. Why else would the ghost child be so possessive of this particular Mom? Throughout the film, the Mom was leaving her little girl alone--the child feeling abandoned at school and at home. While the film didn't make that at all clear at the end, I tend to lean in that direction as the best explanation.

A clever point of view indeed. However, I'm not so sure about that. For example, she says "I am your mother" but is that what she really says in the original script? I mean, it's curious, because in the Spanish dubbed film she says literally: "I will be your mother".

From my point of view, Yoshimi decides to protect Ikiko by accepting the ghost girl. Huge dilemma. And at the very end of the film we can hear Ikiko's voice saying (more or less) that her mother always protected her.

In fact, the ghost girl was just pointing she died inside that big tank of water, but in the lift scene she embraces Yoshimi shouting "mother!" (in Spanish).

I don't know really but it was a good point.

A lot of the responses here have pretty much disproved that theory, however the ghost girl definitely has a distinct attachment to the Yoshimi. Maybe it's because Yoshimi has experienced similar neglect as a child (flashbacks point to this) and understands how it feels, so the ghost girl has a special affinity to her.

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