Tsugaru Shin'uchi is a half-human, half-oni hybrid, the result of experimentation. Originally a human member of a specialized oni extermination squad in Meiji Restoration Japan, he was captured and injected with oni blood by the antagonist Moriarty. This transformation granted supernatural abilities but carried the constant risk of his oni blood overwhelming his humanity, leading to complete loss of sanity. To delay this transformation, he regularly ingests the saliva of the immortal Aya Rindo, which temporarily halts the progression. He sports dark blue hair and distinctive blue stripes along his arteries, visible signs of his oni heritage. His typical attire consists of a blue overcoat matching his hair, a white shirt, black pants, grey gloves, and brown boots, which he removes during combat for enhanced mobility. Following his transformation, he worked as a circus attraction called the "Oni Slayer" or "Demon Killer," participating in staged fights against supernatural creatures, exploiting his combat prowess while reflecting his resignation to his fate. His life shifted upon encountering Aya Rindo, an immortal woman existing solely as a disembodied head. Recognizing their shared connection to Moriarty—he as an experiment, she as a victim—they forged a partnership. Tsugaru agreed to help Aya locate her stolen body and track down Moriarty, hoping the quest might offer a solution to his condition. In return, Aya's saliva provides him temporary stability. He serves as Aya's primary protector and assistant, often transporting her head in a birdcage during their travels across Europe. Their dynamic blends professional dedication with lighthearted banter; he formally addresses her as "master" yet engages in witty exchanges, revealing mutual respect and camaraderie. He maintains an antagonistic yet complex relationship with Shizuku Hasei, Aya's maid, enduring her physical assaults while occasionally showing underlying concern for her. In combat, he relies on superhuman strength from his oni blood, capable of defeating supernatural entities barehanded with a pragmatic style emphasizing speed and precision. The blue coloration aligns loosely with Japanese oni mythology's "Blue Oni" archetype, though his personality contrasts the traditional calm. His unstable condition differs from Moriarty's other successful experiment, Jack the Ripper, who achieved a stable half-oni state without Tsugaru's ongoing physical deterioration. His character development centers on shifting from nihilistic acceptance of his fate to finding purpose through his partnership with Aya. Solving supernatural mysteries across Europe advances their goal of confronting Moriarty and simultaneously reinforces his will to retain humanity. This journey highlights his adaptability within human society despite his condition and his growing commitment to protecting his companions.