Azumi, also known as the Serpent Undead, is a character from Kamen Rider Blade. She is the human incarnation of the Serpent Undead, a powerful immortal being classified as the Category Queen of Diamonds. In her human guise, Azumi appears as a striking woman with an alluring presence, typically dressed in dark, revealing clothing. This outward beauty, however, conceals a deeply unstable and cruel personality. She is instantly recognizable by her shrill, distinctive laugh, which she unleashes without restraint whether she is blending into a crowd or in the heat of combat, marking her as a highly eccentric and psychotic individual.

Azumi's primary motivation stems from her knowledge of a world-ending secret. She is aware that Hajime Aikawa, the amnesiac hero who transforms into Kamen Rider Chalice, is in truth the Joker, an Undead whose very survival at the end of the Battle Fight would trigger the complete destruction of the world. Driven by a desire to prevent this apocalyptic "reset," Azumi makes it her mission to eliminate Hajime. Her role in the story is that of a cunning and relentless antagonist who exploits the weaknesses of her prey. She takes advantage of Hajime's memory loss following an initial attack, using her subordinate, the Tortoise Undead, to hunt for him and launch further assaults. Her single-minded pursuit even leads her to mistakenly attack a stranger, Ryo Mikami, who bears a striking resemblance to her target.

Her key relationships are largely defined by manipulation and opposition. She exerts control over the Tortoise Undead, a lower-ranked monster she has forcibly subjugated to serve as her bodyguard and enforcer. Her central dynamic is with Hajime Aikawa; she is a predator who knows his true, dangerous nature and stalks him relentlessly. When her plans are thwarted by the other Kamen Riders, Blade and Garren, she shows no hesitation in confronting them directly. Regarding her development, Azumi does not undergo a significant redemption arc or change in motivation. Her narrative journey is a short, focused, and ultimately tragic one. After cornering Hajime during a public competition, she forces him to recover his memories and transform into Chalice. Despite putting up a fierce fight, her downfall comes from an unexpected and humiliating source. Enraged by Azumi harming his friend, an ordinary civilian named Ryo strikes her in the face with a hot taiyaki (fish-shaped pancake) iron. This moment of shock and pain leaves her vulnerable, allowing Chalice to land a decisive attack and seal her into a Rouze card.

As an Undead, Azumi possesses a range of notable abilities. In her human form, she can extend her hair into powerful, snake-like whips to ensnare or strike her enemies, a method she uses to capture the Tortoise Undead. Her true form, the Serpent Undead, is a monstrous creature whose body is adorned with snake-like bones. As the ancestor of sea snakes, she is exceptionally skilled in underwater combat. Her physical arsenal includes sharp claws, a large blunt weapon on her right arm that she uses like a hammer, and the ability to fire devastating shockwaves from her enormous mouth. Furthermore, as a high-ranking Undead, she has the inherent power to command lower-level Undead, exemplified by her control over the Tortoise Undead. After being sealed, her card, known as Absorb Serpent, is used by Kamen Rider Garren to assume his enhanced Jack Form. Azumi's legacy is that of a powerful and knowledgeable villain whose plans were ultimately undone by an unexpected stroke of bad luck and civilian intervention.