Aya Koda is a 10-year-old civilian girl from the Osaka area, recognizable by her short red bob hairstyle secured with hairpins, blue-grey eyes, and her typical outfit: a tan duffel coat with off-center fastenings, a yellow pleated dress, violet tights, and pink shoes. She speaks in a distinctive Kansai dialect. After the deaths of her mother and sister, Aya was raised solely by her strict father. He frequently compared her unfavorably to her late mother and sister, deeming her a "failure of a woman," and enforced karate lessons to instill grace. Despite this strained relationship, Aya idolized her father as a symbol of justice, developing a strong-willed, spirited personality. She self-identifies as a "warrior of justice," practicing martial arts daily and demonstrating stubborn determination, often refusing to take no for an answer when pursuing what she perceives as right. Aya exhibits intelligence beyond her years and maintains optimism despite her hardships. Her first encounter with the Armed Detective Agency occurs during bomber Shōsaku Katsura's attack at a train station. Unknowingly receiving a bomb-laden suitcase from Katsura, she is saved by Doppo Kunikida, who disarms it. Insisting on helping capture Katsura, Aya accompanies Kunikida but is later captured and strapped with a bomb. Faced with Kunikida's dilemma of saving her or a train full of passengers, she accepts potential death for the greater good. Both survive due to Akiko Yosano's healing ability. Afterwards, Aya implies romantic interest in Kunikida, who gently declines. Later, during a crisis framing the Agency, Aya encounters the Hunting Dogs military unit at an airport. She overhears their discussion about targeting the Agency and is confronted by Saigiku Jōno, who detects her connection. Jōno covertly passes her a note leading her to witness Justice Vice Minister Ōchi Fukuchi murdering Jōno and revealing himself as the mastermind. Aya photographs the scene but is discovered by her camera flash. Fleeing Fukuchi, she hides inside a coffin containing Bram Stoker. After Bram awakens, Aya forms an alliance, addressing him as "Bram-chan." She learns Fukuchi controls Bram via a holy sword embedded in his body and framed the Agency. Aya and Bram attempt to escape the airport while evading Fukuchi and vampire guards. Cornered by vampires, they are rescued by Kenji Miyazawa. When confronted by the vampirified Ryūnosuke Akutagawa, Aya devises a plan to remove Bram's sword by tying it to a heavy desk and jumping from a height. Her actions dislodge the sword, freeing Bram from Fukuchi's control and enabling him to halt the vampire outbreak, earning Bram's loyalty as he vows to serve as her knight. Aya subsequently witnesses Fukuchi's defeat and Bram's transformation into Fyodor Dostoevsky due to Fyodor's ability. Aya has no supernatural abilities, relying on karate skills, quick thinking, and bravery. She refuses to abandon others during crises, carrying Bram despite his weight and rejecting his suggestion to leave him behind, stemming from a decade-long personal vow to never flee danger. She plays a pivotal role in exposing Fukuchi's conspiracy and enabling Bram's rebellion. Her namesake is Japanese author Aya Kōda, famous for the 1955 novel *Nagareru*, with the character's life partially mirroring the author's experiences of familial loss and paternal influence. Bram remarks that Aya resembles his daughter, differing only in eye color. Aya claims to be the fastest runner in her class and carries a phone used to access radio stations for Bram.