At the age of fifteen, Asagi lived with her father Naoya, an insurance representative who was investigating a ship containing uranium running aground on a moving atoll. A fan of fantasy novels, Asagi was very interested when her father and his colleague Yoshinari Yonemori discovered an ancient prophecy on the atoll. They also discovered various orichalcum beads on the atoll, and gave one to Asagi. When Asagi touched the bead, it began to glow and emanate energy, actually forming a telepathic bond between Asagi and Gamera, the monster that was contained inside the atoll. Over the next few days, Asagi began developing inexplicable bruises and cuts, all while feeling drawn to Gamera, who was shot down over Mount Fuji. Asagi convinced a cab driver to take her to Mt. Fuji, where Gamera was being attacked by the J.S.D.F. Every injury Gamera received was also inflicted upon Asagi, and she fell unconscious. Asagi was taken to a hospital, and remained asleep for several days. Extremely concerned for his daughter's safety, Naoya Kusanagi discussed her condition with Yonemori and ornithologist Mayumi Nagamine, who was studying the Gyaos creatures that were connected to Gamera. They concluded that Asagi was essentially performing the role of a priestess to Gamera, possessing a telepathic link to him and being capable of reading his thoughts.
When Gamera battled Super Gyaos in Tokyo, Asagi and her father arrived to witness the battle. When Gamera and Gyaos took their battle to the sky, Asagi and her father along with Yonemori and Dr. Nagamine followed them in a helicopter. As Gamera was repeatedly struck by Gyaos' sonic beam, Asagi began developing the same injuries as him. Naoya Kusanagi begged Asagi to stop, but she assured him that she couldn't because Gamera was fighting for all of them. When Gamera and Gyaos plummeted back into the city, Asagi exited the helicopter and witnessed Gamera seemingly be consumed in flames. She held her father's hand and began to pray, as Gamera absorbed the fire around him and launched a massive fireball at Super Gyaos, decapitating and killing it. Gamera roared victoriously before glancing at Asagi and returning to the ocean. Asagi remarked that she could no longer read Gamera's thoughts, but was certain that Gamera would return again if the Earth was ever in danger.
One year after Gamera's battle with Super Gyaos, Asagi and one of her friends went on a skiing trip to Hokkaido. However, Asagi's friend broke her leg and the two of them were forced to return home. When they arrived in the city of Sendai, they found it under attack by the Legion. They were evacuated to the nearby airfield where helicopters were taking civilians away, but Asagi's friend tripped and fell in the commotion. Midori Honami, a scientist who happened to be there, helped Asagi and her friend onto the helicopter. However, the Mother Legion emerged from underground and approached the helicopters. Thankfully, Gamera arrived and attacked the Legion, buying time for the helicopters to escape. As the chopper flew away, Asagi looked back and expressed her concern for Gamera. When Gamera was seemingly killed when the Legion flower seeded and annihilated Sendai, Asagi and Dr. Honami visited the ruins of Sendai, where countless people surrounded the fallen Gamera. Asagi reached out to Gamera, and the Atlantean bead that bonded her to Gamera shattered in her hand. Gamera was surrounded by a wave of energy and was revived. He then flew away to Tokyo to stop the Legion. Dr. Honami noticed that Asagi's hand was bleeding, but Asagi said it didn't hurt. She was more concerned that her bond with Gamera had been shattered. Gamera was successful in destroying the Legion, and left Japan in peace.
Asagi, now eighteen years old, had spent the last several years studying Gamera and the Atlantean civilization that had created him now that her bond with him was broken. She met with her old friend Dr. Nagamine and explained that Gamera was able to manipulate a substance called Mana, the life energy of the Earth and all living things on it. Gamera consumed a huge portion of the Earth's Mana in order to revive himself and destroy the Legion, and as a result it was now depleted, allowing the Gyaos to evolve and multiply all over the Earth. She explained that Gamera was desperately trying to defeat the Gyaos as they overtook the planet, all without his link to humanity. Upon learning of another young woman who supposedly had a telepathic link to a Gyaos-like creature called Iris which had wiped out an entire village, Asagi and Dr. Nagamine tried to find her. Asagi was worried that the girl's bond to Iris was disturbingly similar to the bond she once had with Gamera. They traced the young woman, Ayana Hirasaka, to Kyoto, where she was in the custody of Asukara Mito, a self-proclaimed psychic and descendant of the Atlanteans, and Kurata Shinya, a computer programmer. They explained to Asagi and Nagamine that Gamera was a threat to the planet, defending humanity when it was perhaps necessary for the Gyaos to exterminate it and prevent humans from destroying the world any more. They believed that the creature bonded to Ayana, Iris, was destined to finally kill Gamera so that the Gyaos could destroy human civilization. Asagi was outraged and told them that Gamera was created to defend the Earth at any cost. Just then, a typhoon descended on Kyoto, while Iris approached the city in order to merge itself with Ayana.
Asagi and the others traveled to the Kyoto train station, where Ayana awaited Iris' arrival. When Iris touched down in the city following a brief air skirmish with Gamera, Ayana commanded it to kill Gamera. Iris complied and engaged in a destructive battle with Gamera in the heart of Kyoto. Asagi begged Ayana to let go of her hatred, but was unsuccessful. Eventually, Iris and Gamera tackled each other into the station, where Iris impaled Gamera through his shell with its spear arm, seemingly killing him. Kurata and Asukara were killed by falling debris, while Asagi was trapped under a girder. Ayana's friend Katsunari Moribe arrived and tried to kill Iris with his family's sacred dagger, only for Iris to swat him aside. Iris then absorbed Ayana into its chest against her will. When all hope seemed lost, Gamera got up and plunged his hand into Iris' chest, freeing Ayana. Iris impaled Gamera's hand against a wall and drained his power, preparing to kill him with his own fireballs. Gamera blew off his own hand with a fireball and used the stump to absorb Iris' fireballs, forming a plasma fist that he plunged into Iris' chest wound, killing it. Gamera set Ayana down near Asagi and Nagamine, who desperately tried to revive her. Gamera roared loudly, and Ayana awakened. Moribe ran to them and embraced Ayana, who begged forgiveness for her actions. Gamera, maimed and weakened, then walked out to the ruins of Kyoto to face a descending swarm of Gyaos. Nagamine stated that it seemed Gamera would continue to fight, even if alone, while Asagi declared that Gamera wasn't alone.
- Asagi Kusanagi and Inspector Osako are the two most recurring human characters in the entire Gamera series, both appearing in all three films of Shusuke Kaneko's Gamera trilogy.