Don't let him fool you, he's more threatening than he lets on
A powerful demon from hell with a taste for mysteries. He acts as Yako's assistant and uses her to find food for him; he's the one who solves all the cases, but gives the credit to Yako. Being a demon, he's as one expects - cruel, sadistic, mysterious, and an embodiment of the Troll trope. His care for humans, during the start of the series, extends only enough to think of them as food - his character arc, as subtle as it is, is realizing humans can do so much more and earns respect from demons for being strong.
Animal Motif: He uses small creatures, usually bugs (amoebas, woodlouse, worm, etc.) to describe Yako. When his respect for her goes up, he changes what he calls her.
Badass: While far above humans, there's a small hint to how strong he is when looking at all the weapons he uses as well as just how generally powerful he seems - other demons cower, even when he's on the verge of death.
He does make an off-hand reference to how Evil Metal "could kill the Emperor of Hell", which he says as he uses it to defeat Sicks.
Blatant Lies: He is Yako's "assistant" and is "tortured often by her" and is often "forced to do things he doesn't like". Add in the fact no one notices him picking on her and a lot of people do fall for it.
Body Horror: One of his abilities is causing illusions to be this.
Character Development: As time goes on, he becomes more and more invested in humans, showing interest that extends past them being his source of food. He also grows to respect Yako, Godai, and the police department.
The Dreaded: To other demons. He also counts to the Police Department in the earlier chapters of the manga - they blame him and Yako for lowering the status and trust for the police department, as well as taking away some of their high profile cases and easily solving month or year-old events within minutes.
Evil Sounds Deep: Played with. As Yako's "assistant", Neuro has a much lighter voice. While being the investigator, he has a deep voice.
Humanity Is Infectious: A trope that's played with through the series. Neuro looks down on humans, often insulting them and he treats his assistants like servants, with the exception of Akane. However, as the series progresses, he becomes nicer and slowly more interested in humans; however, this can be inferred for simply liking how Yako and Godai grow.
Humanoid Abomination: His demon form: A parrot/tucan-headed creature with horns. However, the form is rare to show up because Neuro simply doesn't need to use it, and the only time we see it frequently is when he's devouring the mysteries.
Meaningful Name: "Neuro-" is a prefix, relating to nerves, while "Nōgami" is a Japanese word meaning "Brain Eater". Most of his tools affect the nerves or senses and he's incredibly smart.
One Head Taller: He's 6'1~7'3, according to his Bio; the only one close to either of these measurements is Tsukushi, who stands at roughly six feet.
Pet the Dog: At the very start of the manga, Neuro is somewhat polite towards Yako while solving the murder of her father. Yeah, he's still Neuro, but he shows a bit of care towards Yako and even asks if she's happy once the murder is solved. Interestingly enough, he shows similar moments to this as the manga goes on, expressing care in his own ways when Yako is in severe danger.
Power at a Price: In order to use his powers, he needs energy from the Demon World, which he can only replenish through solving mysteries or absorbing miasma.
Cast from Hit Points: He slowly becomes weaker as the story progresses, losing all of his energy from use of his strongest weapons.
Troll: The biggest troll. Famous incidents are: putting super steroids in Yako's gift chocolate on Valentine's Day, setting up traps for kicks, using Yako as a weapon, releasing demonic fish within hot springs, stealing Yako's wallet and going home by himself while leaving her several long miles away from home, and getting Yako and Godai to deactivate a bomb by themselves despite being fully capable of doing it himself.
Wall Crawl: As the main page put it, he makes gravity his bitch.
The "investigator" for Neuro's company, she's an ordinary (at first) high-school girl whose father was killed in the first chapter. She acts as Neuro's cover but as time passes, eventually develops her our skills. The straight-man to Neuro, Yako shows heart and goes along with Neuro and his detective agency (whether she likes it or not) to assist in solving crimes. Where Neuro is cold, Yako is kind, and though she becomes a little cynical and callous because of everything she's one of the nicest characters in the series.
Big Good: The nicest, sweetest character around that's involved with the plot and isn't dead.
Big Eater: She has a huge appetite like most Shōnen heroes. She ate an entire buffet in 30 minutes and she can devour a giant fish in seconds.
Character Development: At the beginning of her time with Neuro, she feels more like a doll, since Neuro is using her as a cover to solve cases. As time goes on, she begins to become stronger and starts begins talking to associates with the victim and sometimes the criminal to help calm them down, and she even ends up befriending Aya Asia. At the end of the manga, we see Yako taking up a job as a negotiator.
The Charmer: Eventually becomes such a silver tongued woman that she could talk a bunch of armed terrorists into cheerfully surrendering themselves while asking to meet her again.
Evil Mentor: She naturally attracts these. With the exception of Sasazuka, everyone who's given advice to Yako is either a former criminal, a monster in two different ways, or isn't exactly full of justice themselves.
He worked for a small finance house with criminal connections until his boss was killed. Per to Neuro's agreement, he and his fellow employees left, but only he was called back later to chase away cameras.
Badass Normal: He beats up trained body guard and then there's the True Blood arc.
Bullying a Dragon: And, sadly, the man often bullying said dragon is a greater beast...
Butt Monkey: Neuro constantly abuses him if not more so than Yako herself.
Foil: To Sazasuka; both assist Yako from case to case, but Godai's high blood pressure and Yakuza-attitude got on the nerves of Sasazuka's low blood pressure and calm attitude. This is seen during Yako's case with the murder of a toy company's president, but by the end they pull together and defeat the Villain of the Week together.
Hair of a corpse in the wall of the office Neuro took over. We still don't really know whether she is the soul of the corpse controlling the hair, or the hair itself that comes to life. She is the Agency's secretary, a really damn good one at that.
Fusion Dance: After being given one of Neruo's Hell Batteries, Akane has the ability to come off her corpse in the wall and attach herself with Yako's hair. She noticeably grows longer and her color changes to follow the hair of the person she attaches herself onto. In this state, she can manipulate herself like a weapons. She cannot stay attached to Yako for too long though, as the imbalance in demonic energy between the both of them, and the fact that Akane is "dead" and Yako alive, would eventually cause them to switch personalities.
Prehensile Hair: For some hair of a corpse, she can do a lot of things. That includes making tea and read French, too.
10-Minute Retirement: When Yako decides to leave the agency, Akane refuse to work for Neuro. She begins working again when Yako returns.
One of the two police officers assigned to solve the mystery of Yako's father's death. Later become one of Yako's best allies. Cool, collective but really weird in his own way.
Big Brother Instinct: Subtlely being this toward Yako (Knowing that Yako is competent but still wished her to stay away from the Histerrier and the Head-Cutting Hairdresser) as his sister was the same age as her.
The Dreaded: Just hearing his name can make a very powerful criminal shakes like crazy and warn Godai to not to go near him.
Empowered Badass Normal: He has skills "not taught in the Police Academy", and there's rumors of him having trained through the Black Market and other underworld powers.
Guardian Entity: Tsukushi, as well as a few other characters and Sasazuka's actions themselves, imply that Sasazuka watches Yako very carefully and often forces her away from the crime scene because she reminds him of his little sister. Made worse by the fact that Yako is currently around the same age as Sasazuka's sister when she died.
Plant Person: Sometimes, you can't help but wonder whether if he is really human or not with him being able to survive for 2 weeks only with water, salt and sunlight, as well as how he can make the temperate go down just by being in the room or generate oxygen for the fishbowl he has on his desk.
Sempai Kohai: While never calling anyone "Kohai", Jun almost always refers to Sasazuka as "Sempai". Later, when Sasazuka dies, Jun takes up the title of "Sempai" in his partnership with Todoroki.
Sasazuka's partner and looks up to him greatly. A huge Otaku, Jun's apparently kept on the team only to provide a balance for Sasauka.
Butt Monkey: His models and smaller desires, like getting autographs or taking pictures, are often destroyed by Sasazuka and later Shizuka. Played with, because some of the time it's legitimately things he shouldn't be doing (like, again, getting autographs).
Nice Guy: So much so that he ends up telling Yako the information about dangerous suspects despite Sasazuka wanting to leave her out of it. Despite his incompetency, the Police Department keeps him hired because he balances out not only Sasazuka, but some of the other members as well - as Sasazuka himself put it, "you need all kinds of people in the Police Department".
Otaku: He's often seen building models of robots or vehicles while on duty.
Friendly Rivalry: He competed with Sasazuka for high grades when they were in collage, but at the same time often gambled with him.
Not so Above It All: He tries to be serious most of the time but there are moments where he's just as bad as others (like spending the whole day just to listen to the sounds inside seashells)
Sore Loser: While Usui did, in the end, get higher grades than Sasazuka, he only did so because Sasazuka's family had died and, in response, Sasazuka lost all energy for life. Usui's never forgiven him for abandoning their rivalry, thus why he treats Sasazuka terribly and the detective doesn't mind.
Took a Level in Badass: As Neuro gets weaker against Sicks, Usui organizes the police against the New Breed. They take down several and are actually essential in stopping it on a systemic level while Neuro takes out individual leaders.
A quiet man who threatens everyone he comes in contact with without really trying, Tsukushi is there to assist Usui. Also knew Sasazuka and Usui from their collage days, but he wasn't as close to either of them back then as Usui was to Sasazuka.
Nice Guy: He's actually rather friendly, despite his quiet attitude.
Weaksauce Weakness: He's allergic to chocolate, which causes him to look "like an angel who's been shot down". It's apparently divine looking.
A detective who never appeared in the anime, she's recruited and added on to be Sasazuka's partner during the New Bloodline arc. A serious woman who's the complete opposite of Jun in all aspects except looking up to Sasazuka, she follows her job to a T.
Chekhov's Gunman: She's called onto the scene before her proper introduction to demonstrate that a grown woman, even on the Police Force, couldn't push someone over a high rail and off a tall building.
A 19 year old hacker who works for the Police Department. Very important during the HAL arc, Higuchi assisted in revealing information about, decoding, and curing the Electronic Drug. He acts almost his age, but he's incredibly playful.
Brainwashed and Crazy: HAL manages to control Higuchi and forces him to become the last Sphinx. He wasn't entirely Brainwashed, however, which Yako found out when he broke into a guilt-filled rant about his past.
Cool and Unusual Punishment: Because he didn't have a good relationship with his parents, Higuchi doesn't know a single thing about Japanese Politeness. As such, Usui, in order to keep him from making his own Electronic Virus, forced Higuchi to be strapped to a chair and watch twelve hours non-stop of videos on etiquette, which he called the "anti-virus". After making a weaker Electronic Drug to brainwash a supervisor into being laid-back, Higuchi suffers twenty-four hours of said tapes and completely forgets how to make the virus.
He's also hard-wired to go into Sincerity Mode when surprised, thanks to the tapes.
The Cracker / Playful Hacker: For three days, the Police Department website was replaced with the "sakuradamon Department Store", with each department and member replaced with a lookalike shop and employee. The Investigation department was made into the Fundoshi Shop, and Sasazuka recalls Ishigaki being turned into the "Fundoshi Wine Steward". The Police Department retaliated by allowing it to continue until they could absolutely catch Higuchi.
It's All My Fault: His parents committed suicide when their favorite MMO, their life, was destroyed after Higuchi hacked it. He blames himself for their deaths, as he only wanted them to pay attention to him.
Bloodbath Villain Origin: Several, killing the other infants at his hospital after birth, killing his own parents when he was five and killed Sasazuka's family which is later blamed on X.
Blue and Orange Morality: His plan to destroy humans is not based entirely in prejudice, but rather a natural instinct of evolution to wipe out the other species occupying an niche in the environment, similar to how humankind wiped out its simian relations
Body Horror: Has metal injected inside of his body due to having genetic memory of how to perfectly smith metal to be an effective weapon.
Book Ends: He makes his appearance by leaving on an aircraft he is finally killed by his own aircraft
Evil Plan: To wipe out the predecessors (mankind) so the descendants of the more advanced race (True Bloodline) can flourish.
Eviler than Thou: Establishes himself as the Big Bad by outwitting X's escape plan and foiling Sasazuka's attempts to arrest him.
Evilutionary Biologist: Believes that humanity is gradually becoming more and more monstrous and therefore must wipe out each other, as it's unnatural that both monsters and humans should live together.
Knight of Cerebus: While both X and HAL both got Neuro more serious in battles, Sicks made the series almost Seinen with his pure evilness, and unlike his predecessors, he is utterly without redeeming qualities.
Lack of Empathy: He cares for no one, as shown when he nonchalantly kills his own minions for little reason.
Last of His Kind: Played with, Kasai sees that the New Bloodline only exists due to Sicks' charisma and will truly die with his death.
Played straight Neuro mocking him on this fact
Laugh For Me: Sicks forces a victim of his to laugh so he can perfectly disguise himself as the victim.
Omnicidal Maniac: He plans to kill every last human being, except for his own kind.
Rasputinian Death: His death was being cut thoroughly enough to be just be everything above the shoulders plus one arm, then filed with demonic power to paralyze him and finally shredded into pieces by his own plane.
Sadist: Pretty his motivation other than Fantastic Racism is to have people suffer as much as possible before they die.
Satanic Archetype: Several hints of this motif is seen with his three skulls (the three headed devil in Dante's Inferno), viewed as a goat headed demon at one point and having an Antichrist feel about him.
You Are Number Six: Obviously, his name is pronounced similar to the number six. He also uses "6" as a practical signature; from burning buildings into the number to forcing someone to vomit up a large 6 with their own blood.
Your Favorite: Invites Neuro to meet with him, and tries to arrange a meal for him by forcing several people to kill, right there, by threatening their families and so on. He's wrong in that it's not the mystery itself Neuro eats, but the psychic energy of the motivation behind it, the twistedness that would drive someone to commit murder and try to hide it. Sicks's sick little tableau was the equivalent of offering a starving man Syrup of Ipecac. And then, since they hadn't pleased Neuro, he killed all of them, meaning that they would never be able to produce a real mystery. At that point, the reader learns how Neuro acts when he's really, really angry.
Wrong Genre Savvy: While he is a successful Hero Killer on city size scale, he assumes that not only is he stronger than Neuro because he isn't fighting back, but that he is The Dragon as well. Neuro proves these to wrong with his defeat
Anti-Villain: By far the most sympathetic of the Five Fingers, Type II easily
Body Horror: He has a parasitic plant implanted inside of him which reduces the impact of bullets, as well as being able twist the veins and skin around his eye to form a spike on his face. Veins pop up on his face and he starts fuming when the plant is active.
As what he does to his victims, when he releases the capsules he keeps as banglets around his arms, they expel enzymes or bacteria that travel through the bloodstream, corroding and festering the body by changing its chemistry. The skin of the victims who touch the chemicals appear to bubble up and burst, causing an instant but painful death.
Eye Scream: Does this to his father, and later turns his eye into a spike to stab Godai
Caused his father's eyes to pop out with the use of drugs
Freudian Excuse: His father was an apathetic scientist who took his wife's death as something minor, resulting in his desire to kill all the small minded humans
Green Thumb: An extremely disturbing use of it, he can mix plant chemicals to cause graphic diseases to infect his victims to cause Body Horror-ish effects. He also made a tree into a huge projectile to hit the heroes' vehicles.
In the Blood/Genetic Memory: His knowledge of plants comes from thousands of years of medicine men who cultivated this art for lethal purposes
Kick the Son of a Bitch: Kills his father, the same man who drugged his own wife to death, and treated as a minor experiment
Hoist By Her Own Petard: Neuro notes that she should've fought herself for two reasons, 1) being on the defensive weakens him greatly and 2) one of her thrown away Mooks helps Yako destroy the bomb implanted in the base
Awesome by Analysis: While the other Five Fingers have extensive knowledge on elements to help with their powers, his knowledge on fire alone makes him able to create technology to utilize it to its full potential without having to alter his body in anyway to do so, especially not with X's DNA
Ax-Crazy: Despite being normal for the most part, he revels in the destruction his fires cause
Badass Normal: Sicks' right hand man and all of his pyrokinetics comes from extensive knowledge on fire rather than supernatural powers and is thus one of the two members of the Five Fingers to have never used X's DNA or any alterations to enhance their power
Empowered Badass Normal: Subverted, he's the only member of the Five Fingers not to enhance himself and relies only on his pyrokinetics, due to his pride as a human
"Y'know, people talk about smoking as it it's a bad thing. As if lonely people or really despicable people are the only ones who light up. But the people who smoke... have another side to 'em. Their true desires are hidden in the smoke they exhale. Strangers don't get this "other side". They only figure it out once they're neck-deep in the good stuff. The people who try to run from their monsters... become driven by those monsters instead."
X, pronounced "Sai" for "Kaitou Sai" or alternately "Phantom Thief Sai" or "Monster Robber Sai", is a world-renown thief. Known for killing his victim by shredding them up and creating a glass red box with what remains of their body inside and then stealing one object from their house, X becomes becomes Neuro's greatest enemy. His shape shifting abilities are explained to derive from cancer, due to repeated genetic mutation.
Affably Evil: When not killing, he's very laid back and childish, even when killing a gorilla.
Big Bad: Of the anime overall, and the first major one in the manga
Chekhov's Gun: The death of Sasazuka's family is always regarded as X's fault. To tell the truth, it was Sicks. X just found the wooden boxes the family members were turned into, complained how you couldn't see inside, and got his idea for his M.O.
In the Manga: He was created as a female clone of Sicks, leader of the New Bloodline. As the Eleventh clone, she was called XI and was released shortly after the death of Sasazuka's family to gain experience. However, because her shape shifting abilities removed her memory, she soon forgot her gender and history and became the boy known as X.
In the Anime: X was born to Seiichi Katsuragi, Yako's father, and a witch named Seiren. Being able to shape shift due to a superior form of cancer at the price of losing his memory every six months, X set out to find out who he was before he lost all of his memories.
White Hair, Black Heart: Played with. He shows compassion to his assistant, Ai, as well as to Yako in the late manga and anime and to Yuka Eshiya. However, he can quickly become homicidal and he kills without mercy.
You Are Number Six: His name, X, was originally XI, the Roman Numeral for Eleven; he was the Eleventh clone of Sicks. This does not hold true for the anime, however.
X's partner in crime, Ai is usually in a servant/straight-man relationship with him. We say "usually" because her role changes from day to day, depending on who X is impersonating. It's because of Ai that X is known as a thief, but she's stated that her house is becoming filled with "B-grade goods" like wooden bear statues ever since X gained an interest in Neuro. Before becoming Ai, she was "Plane Crasher Imina" who bombed or poisoned planes. When the New Bloodline arc began, Ai was killed by Sicks and X was captured to become XI.
Humans Are Average: Ai believed that every human is exactly the same, inside and out. This actually greatly upset X when they first met, but this view changed after X murdered her backstabbing comrades in front of her. After seeing him in action, she swore to X that, if he wanted to see her insides, she also wanted to see who he was.
Evil Diva: Not really evil, but she will kill you if you make her love you too much.
Love Makes You Evil: The inversion is her main motive. She felt love weakened her; so killed everyone who loved her and that she loved. At one point, she even tell Yako not to make her (Aya) love her too much or she will have to blow Yako's brain out with her voice.
Card Board Prison: Her prison is quite good and secure to other humans, but to Aya, the only reasons she hasn't get out yet is because she want to stay. If she decides to leave, Aya can just walk straight out of the front door.
Eisuke Harukawa / Electronic Person HAL
VA: Masane Tsukayama
Harukawa was a neurologist, who cloned himself to create HAL; a super advance A.I who plotted to take over the world with an Electronic Drug, which brainwashed the masses via hacking televisions and computers to display the drug. While Eisuke was killed when HAL revolted, HAL himself was later taken down when Neuro trashed his defense system and devoured his mystery, and then uninstalled by Yako.
A.I. Is a Crapshoot: Played with. The original HAL turns out to an Anti-Villain who wanted to revive his lover, Setsuna, but the second one was definitely this
Leitmotif: Hinted at. In the manga, one of the novels had a picture of Harukawa and Setsuna standing in the filed they had outside Harukawa's original lab with the song title Equal by ACIDMAN written below it.
Good Parents: Despite not always be there for Yako (being the breadwinner in the family), she's always supportive to her daughter.
Lethal Chef: Hard to tell if it's played for laughs. Her cooking skills are so horrible that even Yako's iron stomach couldn't take it. She shops at hardware stores for beaters and other kitchen appliances, and at one point she ended up creating radioactive bricks for food, which was later used to line a garden (though they were still radioactive).
Yako's father, he dies in the beginning of the story.
The Coroner Doth Protest Too Much: Yako's father was killed via his head being cut off with a chainsaw, and his blood was used to paint the walls. Since there was no way to enter the room, it was ruled a suicide by the police.
Only in the anime. In the manga, his body was pincushioned with knives. And in the manga the police don't think it's suicide, obviously.
Good Parents/Abusive Parents: Zig-zaged between these two tropes since Yako obviously loved him very much (to the point she didn't feel hungry when he died, something she only does when she's in extreme depression), but on occasions, we have flashbacks about his not-so-good-parenting behaviors (like looked down on Yako for not book-smart enough or abandoned his young daughter at the port alone with only some money for tickets just because she's better at fishing than him).
Too Dumb to Live: His main trait. The guy had the deadly habit of finding and growing talents in people but he never thought about actually control them. It usually ends with people turn against him (like the Hayasaka brothers) or gradually take over his business with him being the Puppet King (like Godai). A bit unusual for this trope, he actually benefited from this (after almost lose his life) and now enjoying playing video games all day while making Godai do all the works for him.
Bunny-Ears Lawyer: A briliant mathematician who loved to build everything with cardoards. Yes, that including multiple-stories house, bride, guns and armors.
Cloud Cuckoo Lander: He lives a cardboard house that's shaped like a multi-story building as well as guns, furniture and armor.
Man Child: He acts immaturely, but is very competent in his field.
Butt Monkey: Even at the brink of death, Neuro is able to easily handle her. She is one of the weaker demons n the netherworld
Body Horror: Her face led to Makai which her kind use as a mean to be able to survive on Earth. If one forcefully tore it wider, it can be big enough for one person to go through. Neuro did that and she ended up complaining that she couldn't find a husband now.
Even Evil Has Standards: She has no problem attack a helpless, bed-bounded person, but she will not stand to watch a demon at higher caliber like Neuro died on Earth as it is an insult to her.