Aya Rindo is a 962-year-old immortal woman perpetually trapped in the body of a 14-year-old girl. Her long black hair, amethyst eyes, and pale skin contribute to a youthful yet enigmatic presence. Discovered as an orphan, she was raised by the sorcerer Douman, who granted her immortality through a forbidden ritual. After Douman died from a rival's poisoning, overwhelming grief drove her to repeated suicide attempts, all thwarted by her immortality, forcing her into eternal acceptance. Centuries before the main events, she cleared the Hasei family of false criminal accusations during the Edo period, securing their unwavering generational loyalty. This bond led her to live discreetly with the family, forming a close attachment to Shizuku Hasei, a descendant who became her devoted maid. During this era, Aya displayed a flirtatious nature, pursuing romantic relationships primarily with women, including a mystical figure from Ming China. Approximately one year before the central narrative, she was attacked and decapitated by Jack the Ripper—a half-oni hybrid created by Professor Moriarty. Jack's oni-derived powers inhibited her regenerative abilities, leaving her alive only as a disembodied head. Confined to a birdcage carried by Shizuku, she initially sought Tsugaru Shinuchi, another half-oni, believing his oni attributes could finally kill her. However, deducing Moriarty's responsibility for both their conditions, she forged a pact with Tsugaru: they would journey to Europe to find Moriarty, recover her stolen body, and achieve their respective goals—her death or his cure. Her personality merges calm intellect with moments of emotional transparency. She exhibits sharp logical reasoning and deductive prowess, rivaling figures like Sherlock Holmes in solving supernatural mysteries. Centuries of existence foster a pragmatic, often deadpan outlook, yet she shows kindness, such as sympathizing with the oppressed werewolf Nora, allowing her escape due to shared experiences of confinement. Lighthearted traits emerge, notably a strong fondness for food, though her head-only state prevents consumption, leading to comedic frustration. Functionally, her immortality grants agelessness and near-invulnerability, though decapitation renders her dependent on others for mobility. Her disembodied state offers stealth advantages, enabling concealment in tight spaces. She possesses healing saliva, administered through kissing, which she uses to treat Tsugaru's injuries—an intimate act reflecting their strictly platonic partnership. Her relationships are pivotal. With Shizuku, she shares profound mutual loyalty, underscored by Shizuku's unwavering service and their implied romantic history. With Tsugaru, she maintains a bantering yet respectful dynamic, their partnership driven by shared goals and complementary skills—her intellect offsetting his physical prowess. Her interactions with adversaries like Moriarty and his associates are marked by strategic cunning as she navigates European supernatural intrigues.