Asuna had a black hair and brown eyes. She wears a school uniform included a orange vest, red tie, white top and blue-green skirt.
As young girl, Asuna has been forced to grow up quickly ever since her father had died, while her mother, a nurse, worked long shifts at a hospital. Asuna spent her solitary days listening to the mysterious music emanating from the cat's-whisker receiver her father gave to her as a memento, accompanied by pet cat Mimi.
In the afternoon of July 12, Asuna was attacked by a strange creature, but was saved by a mysterious boy named Shun. A day later, she skipped class to meet him on the mountain. In July 14, her mother returned home. At school, a new teacher named Ryuji Morisaki was introduced to her class. After class, she heard the news of a young boy's dead body from her mother, but refused to believe that was Shun.
More than a month later, in August 18, Asuna listened to Morisaki's lesson about the afterworld, which was sometimes called Agartha and recognized it was the name of Shun's country. After visiting his house to ask for more information, she left and ran to her hideout, where she saw a boy identical to Shun. As they talked, they were attacked by a group of armed men and a helicopter. The mysterious boy hid in the underground entrance with Asuna, and the two proceeded further into the cave when the cave's entrance was bombed. The two met a Quetzacoatl who had apparently lost its physical senses and attacked the boy. He refused to kill the gatekeeper, giving Asuna his clavis, a crystal, and fought back. The Arch Angels interfered and killed the gatekeeper. The Arch Angel commander captured Asuna and used the clavis to open a gateway to Agartha. The commander and Asuna entered the gateway followed by the boy. Once inside the commander revealed himself to be Morisaki and the boy also revealed himself to be Shin, Shun's younger brother. Morisaki told Shin that all he wanted was to bring back his late wife from the dead. Shin left Asuna and Morisaki.
Morisaki told Asuna that she could go back but she decided to accompany him. They both went into the realm via an underwater entrance. Once inside, they journeyed to the Gate of Life and Death which could bring the souls of people back from the dead, along with Mimi (who had sneaked inside Asuna's backpack).
Along the way, Asuna was kidnapped by a race of monsters called the Izoku. She awakened in a closed area and met a young girl named Manna; they both tried to escape but were not able to. The day began to darken and the Izoku began to appear, but they could only move in the shadows. In their escape attempt they encountered Shin, who helped them but was wounded by an Izoku. Morisaki found Asuna and Manna down the river as well as Shin with the help of Mimi. After Shin collapsed, Asuna convinced Morisaki to take him with them while Manna led them to her village. Once there, the villagers were at first reluctant to help the Topsiders but the village elder convinced the guards to let them in. The elder allowed them to stay one night at the village because they had brought Manna back but they could stay more than that due to past history in that Topsiders always brought bad luck to Agartha. Meanwhile, Asuna checked up on Shin but he yelled at her to leave him alone.
The next morning, Asuna and Morisaki departed from Amaurot by boat, but Mimi no longer wanted to accompany them. Morisaki and Asuna were walking towards a steep cliff when they were attacked by the villagers but they were saved by Shin. Asuna tried to climb down but was too scared, while Morisaki continued on, after trading his gun for her clavis shard and telling her to go back to the surface. Asuna, following Morisaki's instructions to stay in the water during nighttime because of the Izoku, walked aimlessly and asked herself why she came to Agartha; she finally accepted that she came to Agartha because she was feeling lonely. When the water dried up, she was attacked by the Izoku but was saved by Shin again. The two returned to the cliff after seeing the Ark of Life descending. They encountered a Quetzalcoatl who was about to die. Before he died, the Quetzalcoatl sang its song to send all its memories into the world; Asuna now understood that the last song she heard in her world was Shun's song before he was to die. The Quetzacoatl offered to take them to the bottom of the cliff.
At the bottom of the cliff, they both found the Gate of Life and Death and entered it. Morisaki had already made a wish for his late wife Lisa to return, however, her soul required a vessel. Asuna and Shin found Morisaki, who told Asuna she should not have come; she was soon possessed by the soul of Morisaki's wife. To undo Asuna’s possession, Shin destroyed the clavis crystal, despite Morisaki having a knife to his throat. Asuna was now back to her normal self. Asuna headed back to the surface and was seen making her farewell to Shin and Morisaki, who chose to stay behind.
Half a year later, an older Asuna looked out her window at the cliff side where she had met Shun and Shin. She then said her goodbyes to her mother as she hurried to her graduation ceremony.