Aya Natsume is the younger sister of Maya and Shin Natsume, making her the youngest child in the Natsume family. She is a first-year student at Toudou Academy and a member of the Juken Club. Her family background is marked by tragedy, as her parents died by the hand of her older brother Shin when she was very young. She was raised largely by her two siblings, which contributes to her being somewhat naive about the horrific events that have surrounded her family.

In terms of personality, Aya is defined by her passionate and determined nature, which is immediately made clear upon her first encounter with fellow student Souichiro Nagi. When he accidentally crashes into the shower room where she is bathing, she insists that he must marry her, citing a tradition of the Natsume women that the first man to see her unclothed becomes her husband. From that moment on, she pursues him relentlessly throughout the series, despite his vehement protests. Her attraction to Souichiro becomes a central driving force for her actions, though it also creates a rift between her and her sister Maya, as Souichiro is in love with Maya instead. This unrequited love serves as a catalyst for much of her personal growth and her desire to become stronger.

Beyond her romantic motivations, Aya is a character who struggles with her own identity and power. She loved her brother Shin dearly, and because she was not old enough to fight alongside him, she knows little about the true circumstances of his death. This lack of understanding creates a reservoir of built-up anger within her that eventually surfaces when she learns the truth. Her dedication to Souichiro also drives her to increase her martial skill, as she wishes to be able to support him in any way she can. Despite her often single-minded focus on Nagi, she is not without other connections. Masataka Takayanagi is in love with her, but she shows no romantic interest in him. However, she does respect him for his strength, skill, dedication, and typically calm nature, eventually entrusting him with a vitally important object.

Aya's role in the story is significant as she is a member of the Juken Club, the primary opposition to the Executive Council at Toudou Academy. Her actions often propel the plot forward, most notably when she takes the Reiki, her family's ancestral sword, without permission. Her sister Maya is the current guardian of the blade, but Aya believes taking it will allow her to become stronger and win Souichiro's affection. This act causes an uproar within the school and leads to a confrontation between the two sisters. Aya has a unique connection to Reiki, as her power allows her to speak to the spirit within the sword, and she is called its true master, unlike her sister who cannot activate its power.

The most notable ability Aya possesses is the Dragon's Eye, a power she shares with her late brother Shin. This power synchronizes with the spirits of all things, granting her a form of prescience that allows her to see anywhere in the past, present, and possible futures. She can also see and hear the dead, use astral projection, and employ psychometry to learn from inanimate objects. The Dragon's Eye lets her read the energy of everything that exists, allowing her to predict her opponent's moves in combat. However, this power is incredibly difficult to control. It consumes an immense amount of ki indefinitely, and sustaining it for too long can cause her to lose consciousness. When she fully gives in to the Dragon's Eye during a fight, she becomes over-enthusiastic and merciless, unable to stop herself from nearly killing her opponent even after she knows their bones are breaking.

In terms of combat, Aya fights using the Natsume family style, known as Natsume Goushin Ryu. She has an affinity for katanas and is fairly skilled with them, though her first katana was shattered in an early battle. Her repertoire includes specific techniques such as Returning the Scabbard, a staff technique used to disarm an opponent, and Bamboo Grass Summer Rain, a sword technique that transmits attacks through the wind. Throughout the manga, it is shown that Aya and Souichiro share a mental connection that calls to the other when they are in danger. This is likely due to their powers complementing each other, as Nagi's Dragon's Fist can collect and supply huge amounts of ki while Aya's Dragon's Eye can consume an unlimited supply. Her powers fully awakened around the time she entered high school, unlike her brother who awakened his as a boy, though she showed early signs of her ability by being able to hear her dead parents voices when they were speaking to Shin.