The protagonist of the series, Neuro is a demonic detective from hell. He hunts for mysteries within the intricate minds of humans as a source of nourishment. By unraveling the mystery and stripping the culprit of all defenses, he releases the negative energies that led to the crime, which he then consumes. Neuro ventured into the human world seeking the Ultimate Mystery, as those in hell were mundane and repetitive. Upon arrival, he coerced Yako into being his assistant and commenced solving cases.
Neuro is an extremely self-centered, sadistic, and arrogant character who pays little heed to the human elements of the cases, often focusing solely on resolving the mystery as swiftly as possible. This can be disadvantageous, however, since it hinders his ability to relate to or understand the motives of the culprits. He employs blackmail and manipulation to achieve his goals. Neuro delights in tormenting Yako at every opportunity, initially viewing her merely as a slave. Yet, there have been instances where he shows concern for her: saving her from explosions, offering crude comfort after the Hal arc, and even admitting during his battle with X that watching her strive to realize her potential was "very pleasant." Prior to the HAL arc, he frequently urged her to 'evolve' and become more useful to him. His actions suggest high expectations of Yako, expressing disappointment, approval, and even pride at times.
In public, Neuro has Yako act as the detective while he poses as her assistant, despite doing all the investigative work himself. Though Neuro is a demon, he still requires sleep, though not as much as humans. The more exhausted he is, the longer he sleeps. According to Neuro, "Shouki is like oxygen to a demon," and natural hot springs form when excess Shouki surfaces, making them ideal places for him to rest.
Neuro's true form seems to be some kind of horned bird, often revealed when feeding on released negative energy. In the manga, unlike the anime, Neuro isn't limited to consuming mysteries born from crimes; he has dismantled various objects to understand their workings, such as a car, a cellphone, and even a bomb.
As a demon, Neuro exhibits inhuman abilities early in the series, appearing invincible against human weapons, like catching bullets with his eyelashes. However, as the story progresses, he sustains injuries from criminals. Neuro explains to X that his body is gradually becoming more human-like due to the lack of Shouki in the human world and his body adapting to its physics. Despite this, his monstrous endurance ensures his survival far beyond what would kill an ordinary human. Even with his weakening body, Neuro still easily defeated X in combat.
