Kyouko Mejiro, who operates under the alias Anko Uguisu, is a private detective and a former vampire hunter in her late twenties. Her real name is Kyouko Mejiro, and she adopted the pseudonym Anko Uguisu after a traumatic event reshaped her entire existence. She is a young woman with short brown hair, usually tied up, who wears a pair of round glasses that originally belonged to her father. Her typical attire includes a beige trench coat and black gloves, often carrying weapons like a knife or a handgun.

Her personality is defined by a stark duality. On the surface, she can appear straightforward and sociable, but this masks a deep-seated instability and a sadistic cruelty toward vampires. She is highly intelligent and sharp-witted, with a long history of observing and deducing information, but her actions are often reckless, driven by a profound trauma and a notable disregard for her own life. Her primary motivation is a burning, obsessive hatred for vampires, which stems from a singular, catastrophic night that destroyed her family ten years before the main story begins. As a hunter, her method is highly specialized and relies on a particular weakness of vampires: objects they were attached to during their human lives. By using such an object to remind a vampire of their past humanity, she can cause them to disintegrate. She also carries a lighter that belonged to her father, which serves as a potent weapon against the specific vampire who ruined her life.

Her role in the story is that of an antagonist who stands in direct opposition to the protagonist, Kou Yamori, who wishes to become a vampire. She hunts vampires and seeks to prevent Kou from achieving his goal, viewing his desire as naive and dangerous. A key figure in her past is Nazuna Nanakusa, a vampire. Ten years prior, Kyouko Mejiro was a lonely high school student attending night school, where she met Nazuna and they became close friends. Kyouko, fascinated by detective work, would spend time with Nazuna, and the two developed a strong bond, with Kyouko even feeling a romantic attraction toward her. It was during this time that Nazuna revealed she was a vampire, and Kyouko was a candidate to become her servant, though the process was incomplete because Kyouko did not truly love her at the time.

The defining tragedy occurred on Halloween, her birthday. Her father, who had been having an affair with a vampire named Kiku Hoshimi, had been turned into a vampire. On this night, appearing to reconcile with his family, he lost control and killed Kyouko's mother. In a desperate act of self-defense, Kyouko used her father's cherished lighter to kill him, reducing him to ash. This event shattered her, transforming her from the hopeful Kyouko Mejiro into the vengeful Anko Uguisu. She dedicated the next ten years of her life to eradicating all vampires, her former bond with Nazuna severed by the trauma.

Throughout the story, her development is a painful journey of confronting her past and her hatred. Her ultimate plan involves a desperate ploy to expose the existence of vampires to the world by filming her own death at the hands of one, a scheme designed to starve them out. When this fails, she attempts suicide, believing she has lost her purpose. However, she is stopped by Kou, who gets shot in the process. This event forces her to confront the value of life, her own included. She eventually abandons her genocidal vendetta against all vampires, though she retains a specific desire for closure with Kiku Hoshimi, the vampire who caused her family's destruction. She seeks to reconcile with her past, promising to live as a human being rather than a hunter, and even helps Kou and Nazuna by sharing her knowledge to deal with Kiku. Her notable abilities lie not in superhuman strength, but in her keen intellect, her deductive reasoning skills sharpened by a decade of hunting, and her specialized knowledge of vampire weaknesses.