Tsugaru Shinuchi

A young man in his early twenties who identifies as the "Birdcage User" and is half-human, half-demon. He has blue hair and eyes, and blue tattoo-like lines appear on his limbs and the left side of his face, which he hides with a patchwork coat and gloves. He refers to himself with the pronoun "Atakushi."

He was a member of the 6th squad of the Meiji Government's Ministry of Agriculture and Commerce Forestry Bureau's Special Task Force for the Eradication of the Supernatural, known as the "Oni Slayer" 6th Squad, which was assembled to eliminate supernatural phenomena in Japan after the Meiji Restoration. However, he was captured by a professor and subjected to human experimentation that implanted the power of an "Oni" into him, turning him into a half-human, half-demon. After the experiment, he managed to escape from the facility and lived in a sideshow, fighting monsters for survival. During this time, he was recruited by Aya Rindo and became her assistant, heading to Europe.

He is a frivolous character with a very light-hearted personality, often making unfunny jokes and dull stories with a witty delivery reminiscent of a rakugo storyteller, which Aya frequently scolds him for. He lived in the sideshow because he believed that if he were consumed by the demon, he could kill the morbidly curious audience and the ringmaster before dying, which Aya describes as inhuman thinking. Additionally, due to his own encroachment by the monster, he is skeptical about coexistence between humans and monsters.

Having fought monsters daily, his combat abilities are extremely high, surpassing even those of a vampire known as the "King of Monsters." He uses a technique called "Suigetsu," learned from his captain Araya Kuraku during his "Oni Slayer" days, which involves thrusting his fist into a monster's mouth and throat to incapacitate it, then delivering a finishing blow with a precise kick to the solar plexus. Although his deductive skills as an assistant are lacking, he has an excellent memory, having learned ten languages in six months. To prevent the demon's encroachment, he regularly consumes the saliva of the immortal Aya to boost his immunity.