I really liked this movie. I still have an inkling that Diago never really existed. By the end I think that father might have had the same coping mechanism like Kirko. Seeing his daughter jump to her death, imagined that it brought back his fictional son. Plus throughout the movie it looked like he wasn't running on all gears to begin with.
Far Eastern lore regarding the afterlife is quite different from Western motifs, at least today.
What truly links these movies together is, that in Japanese myths, vengeful spirits exist in a central spot of negative energy, that corrupts the purest of souls. Formerly dead people, who had an essence in life can come back to haunt, as so they believe, and at times they can manifest as well.
Daigo was stillborn when Kyoko died, but since the place itself (the hospital) is filled with evil, it gave Daigo powers to be around his family so that in turn Daigo can "feed" the place with new souls. Daigo blamed Kiriko for his and his mother's death, and blamed his father for not caring, so his departing with him had a double purpose: for one, tormenting him for not caring, and two, deliver new souls to the cursed hospital.
I live in the Gordius Apartment Complex, my interior designer was M.C. Escher.