Nikaidō runs the Hungry Bug diner in the Hole, specializing in gyozas. She aids her reptilian-headed amnesiac companion, Kaiman, hunting magic users to find who cursed him. Their method involves placing a magic user's head inside Kaiman's mouth, triggering memories from the human residing in his throat. Nikaidō conceals her own identity as a magic user despite her human appearance. Orphaned as an infant in the magic user realm, she grew up under the care of an elderly couple alongside adopted siblings Yakumo and Kawajiri. They trained her in martial arts, granting her exceptional combat skills. Her magic manifested as a black smoke cube, later refined into a metallic box enabling time travel. During her first accidental use, she encountered her younger self and adoptive parents, altering the timeline and erasing Yakumo's existence. Repeated attempts to restore her sister failed, fracturing relationships until Kawajiri—now transformed into the devil Asu—rescued her. They relocated to the Hole to prevent further use of her dangerous magic. In the Hole, Nikaidō sold gyozas but struggled financially. Defending shop owner Syueron from rioters led to a partnership selling gyozas at his establishment. After vandals destroyed the shop, Syueron departed for the magic user realm, bequeathing the space to her. She rebuilt it as the Hungry Bug. While volunteering to recover bodies, she encountered Kaiman's decapitated reptilian-headed form. She and Dr. Vaux cared for him, forging a close bond. Kaiman became a regular at her restaurant, and they collaborated hunting magic users. During the Night of the Living Death, Nikaidō and Kaiman exterminated reanimated corpses to earn a meat mincer for her restaurant. They encountered En's enforcers, Shin and Noi; Nikaidō fought Noi but suffered severe injuries. Witnessing Kaiman's decapitation, she used her magic to escape to the hospital. Later, she entered a boxing tournament to settle debts but was captured after losing. Kaiman rescued her, and she confiscated prize money from her captor to fund the Hungry Bug. When Kaiman recalled a figure named Risu, they used Professor Kasukabe's artificial magic door to travel to the magic user realm, staying in a magic user's apartment while pursuing the lead. Nikaidō's combat style relies purely on martial prowess and acrobatics, avoiding magic to maintain her concealed identity. Her time magic utilizes a revolver-like device with five crystalline bullets; each firing transports her and those she touches backward in time. Alterations during time travel reconcile at the "firing time," erasing the original version of affected individuals. The device disintegrates after the final bullet is used. Physically, she possesses long blonde hair, blue eyes, and an athletic build. Early attire included dark baggy pants and a Chinese-design jacket, later replaced by green overalls with combat boots. Captivity in En's mansion saw her in minimal clothing due to bandages covering resistant injuries. Devil training introduced heavy armor, later simplified to shoulder guards, a hoodie, and an apron. Post-recovery, she returned to modest overalls. Her demeanor is easygoing and friendly, though she internalizes anxieties. In devil form, she exhibits heightened cheerfulness regardless of circumstances.