Aya Mikage is the sixteen-year-old protagonist who finds her ordinary life shattered on her birthday. She is a high school girl with a typical teenage life, known for her stubbornness and an optimistic attitude that contrasts sharply with the darker forces she encounters. Physically, she is depicted with long blonde hair and light blue eyes, and she wears a distinctive pair of small red earrings that she once gave to her twin brother. Her surname, Mikage, carries the meaning of "honorable shadow".

The discovery of her heritage is traumatic. As a direct descendant of a celestial maiden, Aya learns that she is the reincarnation of a supremely powerful and vengeful being named Ceres. This revelation occurs during a family ceremony meant to test for celestial blood, where her family, fearing the destructive prophecy that Ceres will annihilate the Mikage clan, attempts to kill her. In the ensuing chaos, her father is killed, and her twin brother, Aki, is taken away by the family.

The central conflict of Aya's life becomes her struggle to coexist with Ceres, who manifests under intense stress or fury. While Ceres is driven by an undying hatred for the Mikage family and a desire for revenge, Aya is horrified by the violence and is determined to find a peaceful resolution. She makes a pact with the celestial spirit within her: she promises to help Ceres find her stolen celestial robe, the hagoromo, in exchange for Ceres sparing the lives of her family members, especially her brother Aki. This agreement showcases her core motivation: a deep desire to right the past wrongs of her family and protect the loved ones she still has, even as they hunt her.

Her key relationships are central to her development. Her bond with her twin brother, Aki, is particularly poignant. As the story progresses, Aki is slowly taken over by the spirit of Mikagi, the ancestor who originally stole Ceres's robe and her former husband. This transforms their loving sibling relationship into something far more complicated and tragic. Aya also develops a deep romantic connection with Toya, a mysterious amnesiac young man originally sent by the Mikage family. Despite his initial mission to kill her, he becomes her protector and primary love interest. Their relationship deepens as they overcome his lost memories and his true nature as an immortal being created by Ceres's robe. She also finds refuge with Suzumi Aogiri, a descendant of another celestial maiden, and her brother-in-law, Yuhi, who act as her protectors and allies.

Throughout the narrative, Aya demonstrates significant growth. She begins as a frightened girl on the run, horrified by the power within her. However, she develops into a determined and strong-willed young woman who actively fights for her own destiny. She refuses to be a passive vessel for Ceres, often resisting the spirit's influence and using her own will to temper Ceres's brutal vengeance with her own optimism and compassion. Her journey is not just about survival, but about taking control of her life and future, which ultimately leads her to conceive a child with Toya.

When Ceres takes control, Aya gains a vast array of supernatural abilities. She can fly, teleport, use telepathy, and possess superhuman speed and precognition. Her offensive power manifests as destructive pink energy blasts, and she can create powerful, impenetrable pink energy shields for defense. However, Aya herself has little control over these abilities, as they belong to the celestial spirit residing within her. Her primary power lies not in these destructive forces, but in her own resilience, her capacity for love, and her unwavering determination to forge a peaceful path for herself and her family, despite the tragic legacy she is forced to carry.