Neuro Nōgami (脳噛 ネウロ ,Nōgami Neuro) is a horned bird demon detective from hell who feeds off mysterious/puzzles from objects or people. When he dismantles the mystery it releases energy for him to feed on. He left hell to find a mystery to satisfy his hunger.Once in the human world, he takes on a human form and forces Yako to be his assistant to hide his identity. In public, Yako is seen as the detective while Neuro is the assistant. He treats Yako as a slave and abuses her when he's in a bad mood; though he does keep her from being harmed in dangerous situations and respects her potential in understanding humans. As a person, he is highly intelligent with a sadistic and arrogant manner who only views humans as food. He will do whatever it takes to achieve that goal. As a demon, he is incredibly powerful and practically invincible to any attacks. By using demonic energy, Neuro is able to use tools called 777 Tools of the Demon World and 7 Weapons of the Demon Emperor to assist him in solving mysterious. As he remains in the human world away from Shouki and decreasing demonic energy, his body becomes more human and he begins to be vulnerable to human weapons. He views Sicks an enemy for endangering his food source.
Yako Katsuragi (桂木 弥子 ,Katsuragi Yako) is a 16-year-old high school girl. She meets Neuro when he appears and desecrates her father's shrine while promising to help solve the mystery of her father's death. This meeting quickly leads to a partnership: Yako becomes Neuro's cover, pretending to be an investigator while Neuro acts as her assistant. She has a strong understanding of human psychology able to briefly understand Neuro and X intentions and goals. Despite all she goes through, she is able to quickly adapt to a situation and remain calm. While as Neuro's assistant, she begins to develop her skills in detective work.
Shinobu Godai (吾代 忍 ,Godai Shinobu) worked for a small finance house with criminal connections. He was forced to work for Neuro after Neuro won the bet for the office by solving their boss's death. In the manga however, he was left to be until Neuro found a use for him, in which the demon then forcibly drags the yakuza like man back to the office. He is the jack of all trades for the detective's office and is sometimes referred to as "Neuro's second slave". Godai is quick-tempered and yells a lot, but has also been shown to be methodical, responsible, and trustworthy. He has a fondness for cars and was upset after his was wrecked during the HAL arc. After the Hayasaka arc, Godai runs an information company under Neuro's supervision.
Eishi Sasazuka -
Eishi Sasazuka (笹塚 衛士 ,Sasazuka Eishi?) is one of the two police officers assigned to solve the mystery of Yako's father's death. He was also a former classmate of Usui and Tsukushi in the police department. After Neuro solves the case of Yako's father's death, Sasazuka gives them access to crime scenes, and arrests the culprits as Yako points them out. Sasazuka appears listless and disinterested but he has an agile mind acting very quickly in any emergency, and is an excellent shot. Usually he takes a kind and cooperative attitude toward Yako, but appears to be somewhat suspicious of Neuro. After X slaughtered his family a week before his police exam, Eishi let his career slip away and he disappeared into the criminal underworld. He reemerged a year later, becoming a police officer. He is protective of Yako because she reminds him of his little sister. Recently he has come to learn about Neuro's demonic background despite the display of power he witnessed, is still calm in Neuro's presence. Sasazuka is able to function normally under extreme conditions, which would normally hospitalize most other people. After learning that Sicks was responsible for killing his family, Eishi sets up a trap to slay Sicks' army of modified men, which he accomplishes easily. Upon confronting Sicks, he is killed by X, who had briefly been in disguise as Ishigaki.
Yūya Higuchi (篚口 結也 ,Higuchi Yūya) is a nineteen-year-old detective who works for the police. Although he is far-sighted, he only wears his glasses when using the computer, letting them rest on his forehead the rest of the time. He considers himself responsible for his parents' deaths, because he used his hacking skills to shut down the online video game they were addicted to, which led to them killing themselves. During the HAL arc, he was infected with the Electronic Drug to become one of HAL's guardians, although the infection was only half complete, because he was wearing polarized lenses specially made to protect himself from it. After the HAL arc, he was forced to watch shows on how to be polite drugged with the anti-virus for the Electronic Drug. Though, he still remembered how to make the Electronic Drug, and he comedically used it on a supervisor who was overbearing on him, but this was found out and he was subjected to another session with the anti-virus, though for twice as long, to forget how to make it.
Sai (サイ ,Sai), known as Phantom Thief X (怪盗X ,Kaitou Sai?) is his name formed from the words 'Kaibutsu Goutou X.I' (The X standing for his Anonymity, and the I for 'Invisible'). This is abbreviated into the name 'Kaitou Sai', and thus X is pronounced Sai throughout both the Anime and Manga. Sai is a mysterious human with extreme strength, speed and the ability to alter his own cellular structure; because of this, he is able to change his whole body into any human form available, and also grants him indestructibility. He is seen to survive multiple injuries which would easily kill a normal Human letting him survive from a fall of any height, stab wounds, or crushing. Sai is able to become an electronic being, which allowed him to absorb an electronic being called HAL which grants him HAL's Eye which allows him to control humans temporarily. He believes himself without an identity and, in a desperate attempt to find one for himself, starts taking people apart and studying their insides to find out what gives someone an identity; He kills people and puts them in a transparent box dubbed the red box. He is intrigued by Neuro because he is a demon who he believes will answer his problem. With that reason, he chases and tries to kill Neuro to see his insides. In the anime, Sai was born from a woman named Seiren who was suspected of being a witch. As a baby, he witnessed his mother's death, becoming crushed between stone slabs into a box. This resulted in his personality. In the manga, it is revealed that Sai is an altered clone of Sicks and was actually born as a girl. Her mother was killed after she gave birth to him, and when she was released into the world, she was tracked by the New Bloodline. However, after meeting Ai, who organizes her life and helps him keep the Phantom Thief X character, the New Bloodline lose track of him completely. Sicks manages to track down Sai and capture him again. Her cells were cloned and implanted into the bodies of various members of the New Bloodline. Sai is later revealed to have found her original form of a teenage girl and has gained the ability to read memories. Sicks renamed her XI (eleven).
Sicks is referred to as "absolute evil" by Kasai. Sicks is also the current head of the "New Bloodline," a group of people who distinguish themselves from humanity by the extent to which they have bred 'evil intent' into their bloodline over the last 7000 or so years, and whose aim is to extinguish humanity from the planet. He is the first person Neuro has truly considered to be an enemy, Neuro even said that it was the first time that he'd had such an "unpleasant feeling" once he'd met Sicks in person, as humans are the creators of Neuro's "food", and thus Sicks' goal of wiping them from the earth is challenge enough for Neuro. He signs his impressions by leaving the number six as his signature. Sicks recently watched Genuine become Neuro's servant and was told he would soon be the same as her.