Disclaimer: I own none of the characters, forms, and everything else of Majin Tantei Nougami Neuro. Please don't sue: if you do, then I can't buy the licensed versions when they come out (hint, hint).

This is my first Neuro fic, and I'd love feedback, especially critical feedback. Like Discworld, I've been reluctant to cover this series since I really am in awe of how well it's done and doubt I can measure up.

Well, how well the manga's done. Which is why:

This fic is animeverse, although there are manga elements/plotlines tossed in to cover post-ending.

I hope I'm not the only one who thought this when they saw the anime's ending. I find the anime's portrayal of Yako pretty spineless compared to the manga version, but neither version is (all that) stupid. Manga Yako is certainly aware of Neuro's power but anything but cowed: there's a storyline where she refuses to submit, in fact. They're equals, in a very strange way, and it's a complicated dynamic I admire the author very much for. Anime Yako is more of a follower, a lot more childlike.

There's a scene in the anime where Yako's holding Sai post mystery-solving and the multicolored evil/mystery energy from Sai is turning white and flowing into Yako. Neuro is watching this and he sort of smirks, indicating he knows what's causing this and approves despite the fact this means he's not getting that energy.

Revised 6/25/10.

When he appeared in the office it was as if her father had appeared the day after that horribly mangled… the day after his body had been found, and it had been someone else's body or some other miracle had occurred. It was a miracle that he was alive, and it made her realize how much she'd come to rely on Neuro. Was rely the right word?

On several levels she hadn't actually wanted him to solve her mystery. As long as she was working towards him solving it, like the character in Clamp's series was working towards the spirit problem at that shop, she knew he'd keep his servant/cover story around. She felt like she was needed, or wouldn't be allowed to wander off at least, and because of this Neuro wouldn't wander off, not without dragging her with him.

When she'd gone off to solve her mother's case it had been just like trying to catch the hair murderer. She'd wanted to show off to him. Look what I can do, look what you've taught me to do!

And in the end she was the one who had really solved her father's murder, although Sasazuka was the one who had first made the connection between the red room, the policeman who had entered first disappearing, and Sai. Still, she'd done it, and when she'd held Sai there had been this feeling of achievement, of, of power. That she'd done it.

And that was the moment when she stopped needing Neuro, really, even though he'd been the only reason they'd all gotten out alive.

Neuro was… she couldn't think of anything he'd been wrong about but human emotions, and he knew he would be wrong so he was wise enough to not even go there. So he still hadn't actually been mistaken about them.

There was no way he would have been wrong about sealing the demon world killing him. She'd already noticed that he was getting weaker and weaker. Perhaps that was why she'd needed to prove herself then, so at least he could see it before… Sure, he said he'd become a human, or humanlike, instead of die, but the mysteries were getting more and more dangerous as he got weaker and weaker. She'd known that if he didn't find the ultimate mystery soon the quest would kill him. So she'd wanted to get stronger to help him find it.

So she didn't have to lose her father again, that had been the real reason.

She'd done it, but Neuro hadn't eaten the mystery for some reason and then he'd flown away to die.

It was goodbye, without any doubt. There had been an Aya Asia song on the radio, even. Perhaps he'd done that with some tool, although probably not. It could have just been fate.

So she'd packed up the office. This was the second time she'd said goodbye to a father, really, so…

And then he'd appeared.

The trouble was, she'd spent too much time around Neuro to not look a gift horse in the mouth. To not notice all the clues. Forensics, she really couldn't do, but people, people had become an open book to her, thanks to him.

So she couldn't help but notice that 'Neuro' got humans now. Not perfectly, that edge of not-human was still there, but it was the edge of a human turned monster, not something that was just starting to cross the divide.

She wondered how long Sai had been stalking Neuro, to play him this perfectly. Of course, Sai was good at that.

Yako played along. This was her half-brother, after all, and when Neuro left she'd realized how much she missed his mysteries. Helping the victims, bringing people to justice, helping Sasazuka and his own assistant and…

And the puzzles.

The fact that the power moving her hand when she announced the criminal the first time after he came back was her own? That was a clue she couldn't ignore. And she'd always been able to see what really happened when Neuro ate. Sai's act had to be what other people saw.

Like with Aya Asia, and all the other accused, she'd always had to know why, that was the real question, and it when she asked that she felt the, the understanding. The enlightenment, the pride, that she'd solved the puzzle, that she understood, that she was able to understand because of all that Neuro had taught her. It was a heady feeling, and it made her a little hyper afterwards.

It wasn't until she hadn't been able to get to sleep the night after solving the case of the murder of a famous chef (involving a political scandal, not drugs: the food had been great!) because she was, according to her mother, "bouncing off the walls," that she started to make the connection.

If Sai knew that she knew he wasn't Neuro, then he'd leave and she'd never be able to solve his mystery. And redeem him, but the mystery was what came to mind first.

So she started to study, since she could never do much else after solving a case. It was almost addictive, she found herself wanting more, because there wasn't anything as beautiful as understanding, as the moment calculus equations made sense and stopped being painful and started being as easy as calculating the bill at restaurants (that was as easy as breathing by now), and it was addictive.

Which was a good thing, because Sai couldn't use the tools of the demon world. Accessing the net took actual hacking (and he generally outsourced that to the nice lady with the handkerchief), forensic analysis wasn't done by the Evil Javelin but by a touch, the same ability that let him once analyze the corpses of his victims. She didn't think he knew about the tools, actually, other than Evil Aqua. That one had been hard to miss, but Neuro had usually kept them secret.

She found herself longing for those free-range spy-eyes of his, and even the Evil Butterfly.

It was when the hair treatments she'd done out of respect for the dead, and a sense of debt since Akane's mystery had never been solved actually energized Akane, brought her back to undeath (well, it wasn't really back to life) that she got another clue, and she just couldn't stop thinking about it. It was a mystery, and she wanted, she needed, to solve them. Especially ones this important.

Everything seemed brighter when the fog cleared away and the reason for everything was revealed. At first saying that understanding dawned was a metaphor, but she really was seeing light, wasn't she. Light flowing into her. Light she devoured.

At least they seemed… cleaner, almost, after it. Not Neuro's broken shells of people. So…

Online IQ tests, she quickly determined by looking at how they calculated things, were not reliable. Finding a way to measure her own intelligence over time by herself was difficult, but she was getting smarter. To the point her hand danced across the page as she did the most difficult homework the top-level school she'd gotten into for the food had to offer. And it was boring.

That was becoming a problem, really. Once she'd gotten something it was boring. So she had to look for new things to learn how to do. Helping her mother come to terms with their father's death so that she could stay home, helping her friend get the perfect boyfriend, making Godai the effective CEO of a information-gathering/crooked dealing company to find mysteries for them since Sai couldn't smell them and her own sense of 'here be something interesting' was nowhere near as keen as Neuro's had been…

The company in question had been the one to get HAL all those weapons, so making it mostly legitimate was a good side effect of getting Godai the salary he deserved and so on. And she had Higuchi teach her his own skills. He was one of the ones starting to pick up that there was a difference, but oh well. Sasazuka was keeping quiet for now, as he'd remained silent about Neuro's oddness before his return, and she kept up the charade to show him that this person wasn't the Sai that had killed his parents before he got proof that it was Sai.

There wasn't any warning when the New Bloodline came for Sai, who turned out to be her half-sister. To think that her family had been treated like that because of her father's blood, because Sai's mother, who had been allowed to ape normal to get data, had refused to mate with the high priest there…

Yako might not be powerful, but she was smart, now, Neuro's gift no matter how she had been given it.

And the thing was, as much as Sicks might talk about how they were smarter than humans they were anything but. Humans did absolutely nothing without a reason. If there wasn't some reason it was the optimum choice (instinct, emotion, logic…) they didn't do it. What made them unpredictable to Neuro was that logic played an amazingly small role in the human thought process. Logical actions were logical, and hence predictable.

The New Bloodline had, essentially, one goal. To do evil. The ways they chose to follow this goal were filtered through their other instincts and their life experiences and the various quirks anything human-descended had (Neuro hadn't noticed any significant difference between HAL's or Sai's thought processes and human normal), but then, due to their so-called 'high-intelligence' determined by logic.


On top of that, doing evil included doing evil to themselves, so they would often do things that, especially taking basic game theory into account, not in their best interests. And, once she knew their quirks, they were predictable.

Frankly, given game theory's confirmation that friendly behavior and so-called altruism were the absolute strategies, she thought that their high IQs and powers weren't thanks to their evil but traits they'd been forced to evolve to survive despite the handicap of doing things that, in a social species, were, as Godai put it, "really fucking stupid."

Still, though, eventually Sai had to notice, when one more Bloodline member was taken down not by her powers but by a neat little mousetrap Yako had set up two weeks ago by adding some new additions to the agency's website via Akane.

She'd been thinking about Neuro's willingness to come to the surface world at the time, actually. How he'd given up unlimited power and immortality for tasty food. Even the old Yako wouldn't go that far.

No, he'd figured out everything there was to figure out in a place solely populated by logical beings that understood that pain was simply a method of correcting stupid behavior. Why torture your enemy when that just taught them valuable lessons, like not getting caught?

Neuro had seen death as, as a table. You wanted one when you wanted to eat, but you didn't want one blocking a hallway. He didn't kill randomly, just when absolutely necessary, and when it was necessary he did it with no hesitation: the logical choice. He hadn't understood right and wrong because he'd never gotten why anyone would choose wrong when the wrong choice, as he defined it, was the stupid one. He'd never done wrong: the right choice to make in a situation was the right way to go about it for maximum advantage.

Neuro had been quite literally dying of boredom.

And what good was unlimited power if it didn't keep you from dying? A wrench, or the Evil Javelin: any tool was only valuable if it was useful. His power and immortality hadn't gotten him what he'd wanted, so he'd been willing to lose them in exchange for the chance to get what he wanted. A reason to live, really.

And she'd been his reason to die.

Or was that all of it?

She'd been working on the mystery that was the braineater devil when Sai knocked on the door to her room. The ritual, that she'd nudged Sai into following to keep Sai acting like Neuro and thinking that was what Yako thought she was and so on, was that she'd get dragged off for staying here this late instead of heading to the office first thing in the morning.

There normally wouldn't be a knock.

So Yako looked up from the table, breakfast still untouched, and told her sister, as she started to look like 'herself,' "Welcome home, Katsuragi Sai." She'd need a name besides Sai, but getting papers for the highschool detective's long-lost half-sister that had almost been a victim of the New Bloodline as an infant and so on would be child's play.

"How long have you known?" The question was hesitant, childlike, even though Sai was older than her.

"Since the first time you didn't point out a criminal for me." Come on, sit down, she gestured, and Sai did, still watching her. "How much do you remember?"

"At first I only remembered you, and that you were… part of who I was. I came here, and somehow I knew I had to hide, and I just kept remembering. I've never remembered anything but the shadow of a hand before you solved that mystery and…" She sat down, and Yako pushed the food towards her. She picked up a piece of melon, but didn't eat it. "I've been remembering more and more. A lot of things I don't want to remember."

Yako nodded solemnly, and she could see Sai see that Yako understood, sympathized, cared, and it was really that empathy that had been her power all along. They wanted someone who cared, they wanted to be understood, so if she showed them that she cared and wanted to understand they, like Higuchi and even HAL, would hand her the keys to their mystery, would want it solved.

Words weren't right here. Reminding Sai that her sister cared about her no matter what was.

"They're going to send more. Sicks is…"

"Evil. I know." Yako looked at that piece of melon for a bit, and Sai ate even though 'Neuro' never had. "Don't you dare run away. I don't want you out there alone, Sai. Even if you leave, they'll still try to kill me, and Mother. We're safer together than apart. And you're my sister, it's my job to look after you." Even if she was the younger one chronologically, Sai was so very childlike. The memory wipe when the neurons reorganized was also, given how the human brain worked, a personality wipe in a very real sense. Her power was what Sai's self was building around now. That stability.

"You've really changed."

She'd been acting like her old self around Sai before this. "What gave me away?"

"It was just too perfect. There's an English saying that "once is happenstance, twice is coincidence, three times is enemy action," and Sai could see that Yako even knew the word 'happenstance' like the back of her hand. "Or in this case ally action. He came to the office because of the add on our website that you had Akane add, you stopped us for fish and chips at the bar Godai was about to enter and recognize his old friend, prompting him to show his new power to let Godai know that he was strong, Sasazuka's partner had let their car run out of gas because of that new season of Angel Whip right by Higuchi's location, so the fact his hated regulation cell phone had run out of battery as usual and they wouldn't admit he was there at the front office for security didn't matter, you'd 'forgotten' Akane at the office, meaning she could collaborate with Higuchi on 'finding' a way to decrypt the section of Sick's registry containing information on this bloodline, out of twenty-seven other sections…"

Yako laughed, that 'I'm an idiot' expression and laugh Neuro had used so many times. "Sorry."

"I should have died in that fight. They know I'm not Neuro and they know more about my capabilities than I do." Looking down at Yako's tea, Sai added, "maybe I should have died. They want me so they can get even stronger. And I'm a monster like them."

"Only a half-monster," Yako corrected her. "And my half-sister. And Neuro was all demon, and he was a good person." At Sai's look she added, "By demon standards." She still didn't know what had been up with the demon world almost engulfing earth in miasma, which studies of that strange event had confirmed was hydrogen sulfate. Unless they were trying to terraform the surface.

Yes: terraform. Her theory was that demons were lifeforms descended from the cells that had fled underground to escape the air pollution that had caused what was probably the greatest mass extinction in earth's history. Except for a few rarities like yeast, only the cells that had become symbiotic with mitochondira had survived the corrosive effects of the deadly by-product of photosynthesis known as oxygen.

Given that they had to like high-temperature, high-pressure environments, what with living where they did on earth… well, missions to Venus were a long way off.

"His species did almost wipe out the human race. Including the New Bloodline. And, well, they're demons. I think that if he could be a good person, no. I know you can be a good person, sis."

Sisters. That hit home, and Yako was reminded of the iris agate pendant, connections made by a lightning-quick mind as she watched Sai's face with the patience of someone who was all ears, who wanted to hear, know, understand. A black hole, her mind tossed out that metaphor.

"I hope you're right."

"You're saving the world, you know that right?"

"How much of it has been you?"

"Sai, I can't change shape or heal instantly when I get wounded, or… I'd be helpless without you."

"I really don't think so. You were the one who kept them from getting me."

Yako smiled, showing how glad she was that she had. "With Neuro… he was so smart, and so powerful, I used to be worried that I was pretty useless. I think his whole 'you are my slave and I will make you do things' routine was the demon way of saying that he cared, that he would make sure that I would be able to help him and that he'd protect me. That I was valuable property. It's very odd to realize that his saying I was a wood louse now instead of a paramecium was an incredible compliment…" Sai was lost. "Never mind. Anyway, he needed me because he couldn't get how humans worked. He just couldn't grasp it. You have a hard time with it, but I like showing you how. So… I feel glad that I can save you even though I don't have super powers. But you're learning really quickly, Sai. So I've been looking for other ways to help. I've lost enough family, and I'm not letting you escape again, much let them get their filthy hands on you. So, I'll do my best not to slow you down. Just like with him."

"You're not slowing me down. I spent my entire life trying to figure out how people ticked so that I could understand myself, and you just… it's simple to you. You just get it, and you can move people that scare me that much around like chess pieces."

"You used to scare me. I had to work at this, you know. I think it might be that back then you only wanted to know about you and I wanted to know about them, maybe. But you care about me now. Yes, you do. I can tell. That's why you're learning so fast." She was proud of Sai, and it showed.

"It's because of you."

"And I would have been killed several times by now if it weren't for you. Sisters do things like that for each other. People do. We're stronger together than apart, and that's why the New Bloodline can't win. They're neglecting the human race's greatest force multiplier, and we're," or Yako was, "taking advantage of it. Sasazuka would have died fighting Terra if you hadn't helped… we can't beat them alone. They'd pick us off easily one by one. But together…" And it was beautiful and fascinating and what would let them get through this, and she couldn't wait until Sai understood that Yako loved her and what that really meant.

And Sai wanted to understand.

And Yako was getting hungry again. The New Bloodline, their motivations, their 'evil intent' was too simple. Too meaningless, useless, powerless. Even when they'd arranged elaborate mystery setups as traps they'd tasted like diet food. They could make them seem like something she could sink her teeth into but at the center there was nothing worth the effort.

If Neuro hadn't died in the demon world, he'd very likely have starved to death fighting these morons.

Luckily she was a long way away from the one who had solved all the mysteries of the demon world. And she wasn't using tools that drained her power.

Proximity alert on her laptop, she hit the icon to bring up the cameras. "Sai, you might want to switch, mother's coming home and we need to figure out…" the words 'how to introduce you,' died on her lips. That wasn't mother. A New Bloodline member, from the way they'd just torn the heart out of the chest of the police bodyguard Usui had posted here against her protests (she thought she had protested too much: Usui was suspicious, and Yako was indeed hiding something. Sai.).

This wasn't the one that should have been sent out next. Sicks must have figured out how she'd known what he was doing from what she knew, she'd been too clever and it was going to get them killed!

"Yako?" What's going on?

They weren't demonstrating any powers but strength, she had no clues to identify what they were up against with, they wouldn't have sent anyone they weren't sure could take down Sai (and they knew Sai's weaknesses better than Sai herself…) .

"One of them is outside. And I have no idea what to do." Running was out. That was the obvious choice, and they'd have spotted it and planned for it with that high IQ of theirs.

They couldn't hide, they'd be found. Even Sai.

She found herself wishing Neuro was here. He'd know what to do, have a way out. She'd even take getting covered in that Evil Canceller gunk again, like when they'd tracked the copycat killer that had led them to Sai. But no, that only covered visual, and someone looking for Sai wouldn't just use their eyes.

Evil Blind. That had made Godai and his fellows ignore them no matter what they did. But it was power-costly, according to Neuro, and she had no idea how to summon it.

Well, there had to be a reason he said those words. Incantation? And…

It would have been scary how quickly she figured this out, how it made perfect sense if her scare quotient wasn't being used up by the New Bloodline member about to open the front door.

Sai had tensed, battle ready. "Sai," pay attention, he'd open the door soon. "This won't be invisibility, exactly. He'll be sort of aware we're here, but ignore us and it only works in a ten meter radius. Neuro said it was used for killing enemies and doing searches, he…" he'd held claws to Godai's neck and been unnoticed as a demonstration. "You can kill him, he shouldn't even really register that he's under attack, but hurry!"

It was almost like his power was guiding her arms in the same way he'd used her to point out criminals. Yako stood, and focused, as she firmly stated, "Of the 777 tools of the demon world, I call on the lethargic magic lantern named Evil Blind," without any doubt it would come to her hand because doubt that her power would work was thinking that it wouldn't work was telling the incantation not to activate… It wasn't exactly his incantation, but she had to do the full version, be more specific, obviously, since she wasn't as strong, it wouldn't recognize her call.

Except that it was related to his.

She didn't let herself dismiss it before she saw that Sai had rendered the assassin into small chunks. Oh, and then she blacked out.

When she woke up, she was breathing hard, and didn't go back to normal Sai told her they were safe and she finally calmed down. Yako wasn't panting anymore, but she needed slightly deeper breaths then before, such a slight degree that she wouldn't have noticed if she hadn't looked for it.

Her increased appetite… a lot of her cravings had been for 'brain foods,' but there were also particular amino acids, often sulfur containing ones… She might have to look into supplements.

She wondered for a few moments about the irony of her becoming a 'monster' as Sai became more human, and if she should resent it. Probably the normal thing would have been to resent this, she hadn't even been informed except by Neuro's sparse words about evolution and so on, let alone consented.

If he hadn't caused this, however he had, she would be dead and Sai would be a lab rat for those, those, vermin!

It was probably an effect of the transformation that she was looking at it logically instead of mourning whatever percentage of her humanity she had lost. Or maybe not.

Neuro had been… not her father, but a mentor, a father-figure, twisted as it had been, when she'd needed one. She liked, emotions as well as logic, that she was marked by their time together. That she couldn't forget how he'd changed her life. That he was protecting her even after he'd died doing so.

So she was smiling, not frowning, as she and Akane researched like madwomen to figure out the next threat.

She'd figure this out, she'd make this work, she'd make Neuro proud of her.