The title character of the series, Neuro is a demon detective from hell. He looks for mysteries in the labyrinthine minds of humans for sustenance. When he dismantles the mystery and strips the culprit of all of their defenses, that releases the negative energies that caused the culprit to commit the crime, which he then consumes. He came to the human world in search of the Ultimate Mystery, as all of the mysteries in hell were bland and repetitive. Once in the human world, he forced Yako to be his assistant, and began to solve mysteries.

He is a very self-centered, sadistic and arrogant character that cares little about the human aspect of the cases, most of the time only focusing about getting the mystery as quick possible. This can be a disadvantage, however, as he can't relate to or begin to understand the motives of the culprits. He will also commit blackmail and manipulation to achieve his goals. Neuro enjoys abusing and torturing Yako at every chance he gets, and thinks of her as nothing more than a slave to use at the beginning of their 'partnership'. However, recently, there have been a handful of times where he seems to care for her: saving her from injury more than once by explosions, comforting her crudely at the end of the Hal arc, and even stating during his battle with X that seeing her struggling to find her own potential was "very pleasant". A little before the HAL arc he was constantly telling her to 'evolve' and become more useful to him. From his actions then, it is possible that Neuro expects quite a bit from Yako, expressing disappointment, approval and even pride at certain times.

In public Neuro makes Yako act as the detective, while he acts as her assistant, even though Neuro is actually doing all the detective work. Even though Neuro is a demon, he still needs to sleep but not as long as humans; though the more he is more exhausted, the more he shall sleep. According to Neuro "Shouki is like Oxygen to a demon" and natural hot springs are formed when excess Shouki comes to the surface, so the best place for him to sleep is near a hot spring.

Neuro's true form appears to be some kind of horned bird, often revealed when he is about to feed on the released negative energy. In the manga, unlike the anime, Neuro isn't limited to eating mysteries made by crimes; he has, on at least three occasions, completely disassembled various things to find out how they worked, such as a car, a cellphone, and even a bomb.

As a demon, he shows his inhuman abilities in the early chapters of the series, making him seemingly invincible against human weapons such as catching a bullet with just his eyelashes but as the story progresses, Neuro has received injuries by several criminals. Neuro implies this to X that his body is gradually becoming more like a humans, because of the lack of the Demon's world's Shouki in the human world and that his body is adjusting to the Laws of Physics in the human world. His monstrous endurance remains and ensures his survival longer than what would have killed an ordinary human. Even while his body is weakening, Neuro still was able to easily defeat X in combat.