Alternate Character Interpretation: Irys. A monster who mercilessly torments Ayana with memories of her family being killed, or a tragic case of Ayana's hatred twisting the creature's psyche to the point that it doesn't even know that what it's doing is wrong?
Awesome Music: Kow Otani is at the top of his game here, with several great pieces near the end, such as:
Offense and Defense, when Gamera fights Irys in the sky.
Descendents fo Antiquity, when Ayana is racing to the front of the station to see Irys out in the city.
Kyoto In Flames, plays when Asagi and Nagamine try to convince Ayana that Gamera isn't her enemy, while Gamera and Irys fight each other.
The Red Lotus Flame, which plays when Gamera kills Irys.
The Guardian Beast, which plays when Gamera revives Ayana.
Ending Theme: Gamera 1999, which ADV used as the music for the DVD's main menu.
And, while not composed by Kow Otani, we have Julianna's song, Tell Me Once Again, which plays over the end credits.
Tear Jerker: When you realize that not only is this the last movie in the Gamera trilogy, but also the last film of the Kaiju Heisei era. Everything in the film also counts to an extent, from Ayana's tragic past to Gamera's own treatment despite how he's only doing what he was created to do, and even Osako's life going downhill so much.
True Art Is Angsty: This is the most critically acclaimed film in the franchise. It's also the absolute darkest and most horrific. Of course one should note that the film ends on a note suggesting that we have to have optimism and hope that Gamera can win out over evil in the end.
Unintentionally Unsympathetic: Ayana. Despite losing her family she's thoroughly unlikable throughout the film, expresses no guilt that her monster's killed innocent people, and thinks everything is all about her. Its only after until being absorbed by Irys she sees the true magnitude of what she's done.
Though to be completely fair its clear that the girl has some severe mental problems and at some points she even seems borderline suicidal. It's extremely likely she has PTSD or another form of mental strain/damage.