The baby Iris has an armored, mouth-less head with small black eyes, and several long tentacles which sprout from a snail-like shell. It can absorb the life force of organic matter by stabbing them with its tentacle spears, leaving decayed corpses in its wake. The Behind The Scenes feature on ADV Film's DVD shows it was realised on-screen by a remotely-operated puppet.
The adult Iris is basically a bipedal creature with two hooved legs and retractable sword-like arms, topped by a head that resembles a pointed seashell. Its back is a mass of saw-edged plates, and from its sides spring four tentacles hundreds of meters long. Each of these is tipped with a bony spearhead, from which Iris can fire a sonic beam similar to the Gyaos. Iris can still drain the life force from its victims, and in the case of Gamera it was able to absorb the monster's abilities and produce its own fireballs. Its chest region is also covered in glossy, bioluminescent patches. The center patch can suck things into its body. While Legion is the biggest monster Gamera has ever fought Irys is the most powerful.
Iris begins its life as a stone egg, sealed within a small underground temple in Nara village and guarded over by a family line named Moribe - local legend said it was the resting place of a demon called Ryu-Sei-Cho. (A character explains this name to be linked to Chinese mythology) A stone of unknown origin is said to keep the demon dormant as long as it's not moved.
A young girl named Ayana Hirasaka - left an orphan by the 1995 Gamera/Gyaos battle and blaming Gamera for her parent's deaths - entered the cave on a dare and moved the stone. In the process she and the latest son of the Mirobe line discover both the egg & an amulet, similar to the one that linked Asagi to Gamera in the first film. The egg later hatched, produced an infant Iris.
Ayana gains a link to the creature via the amulet and feels a kinship with the creature, stating at one point "Gamera killed its family too"; driven by total hatred for Gamera, she raises Iris (naming it after her dead pet cat) in hopes that it will become strong enough to kill him for her. It was shown to have an affectionate nature, but this is shown alongside the sinister way it eats and, later in the film, the woodland animals it leaves dead.
The Japanese Self Defense Forces quickly deploys and surrounds Iris as it sleeps, attempting to take it out with machine guns and bazookas. Some of the soldiers are killed before Iris takes to the sky and heads for Kyoto, where Ayana had been transferred earlier. The JSDF sends two F-15s in an attempt to shoot Iris down, but it is able to evade them and almost kills them. The fighter pilots are saved by the sudden arrival of Gamera, who engages Iris in a mid-air battle. The JSDF ends the battle by shooting at Gamera, which slows him down and allows Iris to continue on to Kyoto.
Infuriated, Iris pins Gamera's hand to a wall with its spear hand and begins to absorb Gamera's energy. Its tentacles begin to form plasma balls, when Gamera quickly uses a plasma ball himself to sever his own hand. Iris fires the absorbed plasma balls at Gamera who uses his stump to turn the plasma into a plasma fist. Gamera then uses the fist to slice through Iris' chest, killing it and ending its reign of terror.
Unconfirmed origins and motives
The origins of Iris are not explained, and instead the film presents Chinese mythology as explained by Ayana's adoptive brother, the beliefs of the Moribe clan matriach, the theories of government agent/misanthropic recluse Kurata Shinji and the actions of government agent & cultist Ms Asukara.
Grandmother Moribe states that the Ryu-Sei-Cho is a demonic creature that will destroy the world if it is ever released. In contrast, the mythology that the Ryu-Sei-Cho name comes from suggests that the being is the mythical "Guardian of the South", who will defend against a threat from the South; a tortoise (hinted to be Gamera) is said to be Guardian of the North and Ayana asks if the two Guardians are rivals (this was unconfirmed).