Question (spoilers, maybe)

It wasn't the ghost of Aimi but the ghost of Ai, the daughter of the guy who had the building built. Ai's father did those nasty things to her and when she dared to hit him one day he strangled her. He then mured her body and told the police that she didn't return from school; her vanishing was never solved.
Ai's ghost still "lives" in the building, tormenting the residents and holding them prisoners: She kills everyone who leaves the house and doesn't cross the line at the entrance before midnight, kind of psychological torture that works just fine. Takashi's mother (the one who moved to Okasa) says something like "she loves her home and everything about it so much, she doesn't want us to leave". But I think that's rubbish, Ai just takes revenge, and like so many ghosts it's not enogh for her to kick daddy's butt, she wants to kick everyone's. Perhaps she's mad about the residents for not helping her when she was killed or when her dad went after her every day. Perhaps the older ones all knew? Though that's never revealed int he movie.

The soldiers are ghosts she called. Somewhere in the movie they say it (I think it's Takashi's mom in the playground scene) The soldiers give a hint that the old man was a lieutenant in some war and left them to the enemy to save his own life. As far as I understood it Ai gave ghosts like them the chance to get their revenge on the people in the house, she kind of invited them there. The old man asks the soldiers to kill him instead of his wife, but as one of them says "you get them best when you harm those who they love most, not themselves", that's just the best torture for him they can think of. Every night he has to watch them kill his wife without any chance to do somethin against it...

Aimi isn't a ghost, she is is alive through the whole movie, but at the end she finally gets a chance to get revenge on her father who did just the same things to her as Ai's father did to his daughter. Aimi decides to stay with Ai at the end, strange friendship but she wants to...

Hope that helped, though a bit late :-)

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Suicide is the most sincere form of self-criticism.

Ai's ghost is lonely, bitter, vindictive, manipulative and wants to make other people suffer the same way she did. She may have not been like that at all when she was alive, but that's what she became to be when she died and became an evil spirit, well at least that's what I think.

Living in that haunted apartment alone does funny things to whoever becomes a tenants there, especially those who stayed the longest. All those who lived there are being psychologically tortured on a daily basis, and some of them are very mentally disturbed, like those poor married couple ewwww!!

Like how the little boy will suddenly strangle his sister (the same as what Ai went through by her father's hands) or even the fact that Aimi's father started doing those nasty things to his daughter. We do not know for sure if he ever did that before their moving in, and it could be because of his drinking problem, but that is not necessarily the case. It could simply be the effect of living in that apartment. I could be wrong, but I don't think so :)

The fact that the little kids mysteriously become gray-haired and stopped aging (actually they seem to be barely alive) may be because the ghost girl Ai was bitter and envious of them because she never got to live and grow up.

In the end the other tenants got to leave the place and the curse broke, which is a relatively bright and happy ending, of course except for Aimi who remained to stay with Ai. That probably happened because of Aimi avenging for what her father did to her, somehow that set Ai's spirit free - or not? I wonder what happened to Ai's father hmmm?

This is what I like about Asian horror, only some of the plot's major parts are explained, many are left for your own imagination.

By tha way Giggle-Goth, you kept repeating "Takashi" but I think you mean "Takumi"? Takashi is the boy who's about the same age as Aimi, while Takumi and Ai (she has the same name as the ghost girl) are the gray-haired little kids. Japanese names can be confusing if you are not used to them :S