Hazuki Kakio, often called Zuki, is a main character known for her role as a martial arts expert and a physical education instructor at the same college where Sorata Muon teaches. She is easily recognized by her short green hair and blue eyes. Her physical prowess and combat skills are her primary assets, making her the agile fighter and combat specialist of her team.

Personality wise, Hazuki is defined by a complex psychological condition described as multiple personality disorder. Existing within her are several distinct personas, which range from a mischievous, prank-playing child to a serious and brave fighter. This fractured sense of self is a central aspect of her character and a significant source of personal struggle. Despite this inner turmoil, she displays a quirky and playful side, perhaps most notably through her odd habit of suddenly appearing in unexpected places like bathrooms or washing machines while wearing very little clothing.

Her primary motivation stems from her deep loyalty and affection for Sorata Muon, the master thief known as Mouse. She is one of his three devoted female servants or assistants, living and working alongside him while helping him execute elaborate heists. Her role in the story is largely defined by this service and a specific, pivotal incident involving her psychological condition. The incident occurs when Hazuki foolishly decides to test a machine designed to erase specific human memories. The machine malfunctions, and her multiple personalities are scanned and fragmented within its computer system, trapping her in a debilitating and emotionally fractured state where she is completely isolated from the outside world.

This event forces Sorata to enter the digital realm of the machine to rescue her. Inside, he discovers the traumatic secret behind her condition, learning that her family had locked her away because of her disorder and abandoned her, leading to her fragmented sense of self. The key to saving her lies in Sorata gaining her trust by making a heartfelt promise not to leave her as her family did. Through this act, he is able to help her merge her many personalities into a more cohesive whole. This arc not only showcases her vulnerability but also solidifies her relationship with Sorata, as she becomes eternally grateful for his rescue and his acceptance of her true self. Her abilities remain focused on her physical combat skills, which she continues to employ as a loyal member of the team, but her ultimate development is emotional and psychological, overcoming a past of rejection and isolation to find a sense of belonging.