Lady Aone is a character from the Onmyoji franchise, specifically appearing in the live-action film adaptation rather than the later anime series. She is an immortal woman who was once the lover of Prince Sawara during his lifetime. After Prince Sawara was wrongfully accused of treason by his brother Emperor Kanmu and committed suicide, swearing eternal vengeance, Aone was rendered immortal by consuming the flesh of a mermaid approximately one hundred and fifty years before the events of the story. Emperor Kanmu then appointed her to guard the burial mound where Prince Sawara's vengeful spirit was sealed away, ensuring it would never reawaken.

Aone’s background is marked by tragedy and duty. Having lost her beloved prince, she carries the weight of both her enduring love for him and her responsibility to contain his spirit. Her immortality sets her apart from ordinary humans and gives her a perspective shaped by long experience and loss. She is resourceful and resilient, willing to put herself in harm's way to fulfill her role. When attacked by enchanted imperial police, she is severely wounded but remains unharmed due to her immortal nature, demonstrating a calm acceptance of her unique condition.

Her motivations are primarily driven by her sense of duty and her lingering feelings for Sawara. She works alongside the onmyoji Abe no Seimei and the court noble Minamoto no Hiromasa to protect the capital from the schemes of the rival onmyoji Doson, who seeks to awaken Sawara's spirit to overthrow the emperor. Aone reveals a deeper understanding of fate, telling Seimei that the stars have foretold that he and Hiromasa are destined to be the two protectors of the city, neither able to survive without the other. This suggests she possesses some knowledge of divination or prophecy.

In the climactic events, after Hiromasa is mortally wounded, Aone makes a pivotal decision. She proposes that Seimei perform the ancient rite of Taizan-fukun, a ritual that can resurrect the dead, and offers to sacrifice her own immortality and life to power the ritual. This act demonstrates her profound selflessness and her willingness to give up everything for the greater good. After Hiromasa is brought back to life, Aone’s spirit speaks through his body to confront Prince Sawara’s vengeful ghost. She appeals to their past love, convincing him to release his hatred and find peace. Ultimately, she passes into the afterlife alongside Sawara, finally reunited with him after centuries of separation.

Aone’s most notable ability is her immortality, which she gained from consuming mermaid flesh. This makes her resistant to physical harm that would kill an ordinary person. Beyond this, she appears to have been entrusted with the guardianship of a significant spiritual site and possesses enough knowledge of onmyodo and ritual to understand and propose the Taizan-fukun rite. Her ability to speak through Hiromasa’s body after death suggests a lingering spiritual presence or connection. Her true strength, however, lies in her emotional perseverance, her capacity for sacrifice, and her power to reach the heart of a tormented spirit through love and memory. Her key relationships include her past bond with Prince Sawara, her alliance with Seimei and Hiromasa, and her long-standing service to the imperial line under Emperor Kanmu’s command. Through her sacrifice, she achieves both redemption and reunion, playing a crucial role in averting disaster and bringing closure to a centuries-old curse.