I don't own Majin Tantei Nougami Neuro, but I do so enjoy the series, and pay tribute through fan fiction.

For those of you greatly confused, I've updated this with a rewrite (5/29/2016). I wanted to get Neuro a bit more in-character than I had had him, and expand a little more on the idea behind the title. Still a one-shot, but now a much longer and more satisfying one. Not sure where exactly it might fit in the manga timeline (not the anime, because while I love it, it completely nuked the beautiful character development from the manga). It goes in there somewhere. Probably.

"Human emotions are worthless to me," he stated dully, flipping another page in the book he was skimming.

He could feel Yako's eyes boring into him, but made no move to meet them.

"I leave you to handle the matter of unraveling the human heart to draw out the mystery for a reason. Those emotions are too unreasonable and unpredictable for a being such as myself," he continued, though bored with the direction their current conversation had taken. He stifled back a yawn caused by a combination of increasingly uninteresting reading material and lack of a good meal as of late. "Thus, human emotions are your field, and I need not consider them."

This hadn't been the first time she had tried to talk to him about the aspect of their work that he couldn't and didn't care to understand, and knowing Yako, this likely wouldn't be the last. Their last case seemed to have taken something of a toll on her, though he couldn't understand what about it should have left her in tears, and had thus launched her into speaking about the topic of emotions as humans saw them. He supposed as a human herself the topic was of importance, but it was not of importance to him, and he saw no reason for the conversation to continue. On matters like this, his word was final. She was persistent for a louse who was supposed to know that.

He heard the click of her chopsticks being set aside, though he was certain he still smelled food left in the tray. The next moment she was standing, and he finally looked up when she snapped, "Well, maybe it's about time you did consider them."

Surprised and intrigued by the outburst, he found himself being presented the image of her back. That image was accompanied by one of the strangest actions he had ever seen – her meal, half eaten, being discarded in the trash.

How unusual.

"You've never thought about why mysteries here are so different, have you?" she continued with something like a laugh, though he could not understand what might have been amusing. For some reason, she saw fit not to turn around. "I mean, for someone who ate all of the mysteries in the demon world, I thought you would have figured it out by now."

His eyes narrowed, half at the implied insult, half out of intrigue. She was tip-toing the line he labeled questioning his intelligence, which was not something anyone got away with, least of all her. What kept him from going to her side to remind her of that via her skull was his curiosity at what she might say next, and the small fist gone white and shaking with tension at her side.

She became like this when the ones who held mysteries used methods she greatly disagreed with.

Currently, then, she was greatly disagreeing with him.

"Emotions, Neuro," she concluded, her voice quieter than before. He leaned forward in his chair. "What makes human mysteries so appetizing to you are human emotions. They're like…like seasoning. They flavor the mysteries like herbs and spices flavor food. Without emotions, the mysteries here would be just as bland as the ones you complain about."

Emotions…as seasoning?

"So, maybe you should start considering people's feelings. You don't have to understand them; just acknowledge that they exist and that they affect people."

Her tone begged something as it cracked an octive higher, but Neuro was more interested in the logic behind her argument.

It was sound, of course, and he expected no less from his number one slave. He kept her around precisely because she could understand that which he could not. While he could easily unravel the how, she was much more suited to the why. Matters of the human heart he did not comprehend as a demon, and when Yako had interrupted after his second meal with her, during the case of her father's murder, to ask why the man had done it, the meal had become that much more appetizing. That moment had established his method of work in the human world, solidified that she would make the perfect cover.

Now that he thought about it, the puzzles that had required the most work from Yako had also been the most delicious and satisfying. They had been more deeply tied to the things that flavored them. Emotions, then, were things he should endeavor to pay more attention to. How intriguing.

He felt as though he was missing something about this conversation, though. Something Yako would pick up on quickly that he would overlook, because it involved those complexities he couldn't understand.

Something that made him wonder why they were having this conversation now.

Suddenly the door to the office slammed shut, and he blinked in surprise when he realized Yako was no longer standing there.

"Well, maybe it's about time you did consider them."

Neuro stared at the door for a bit longer, and then leaned back in his chair, one hand coming to rest on his chin thoughtfully.

...

Yako did not appear in the office the next day.

He did not want to seek her out. He refused to stoop to chasing down a creature so beneath him. Besides which, he was busy.

For once, he was trying to figure out the why.

He knew Yako's departure last evening was for reasons she hadn't stated but had clearly intended for him to understand. It was frustrating, because he had said it so many times before and she seemed to understand well enough that he did not understand the human heart. It was much more complex in ways that made absolutely no sense to any logical being. He was a logical being. Her display was not.

The conversation itself was clearly meant to inform him that emotions were something he should consider despite his inability to understand them on human terms. He did of course feel emotions himself, but they were much simpler in comparison to the complexity of his food source. There were clear reasons for his own emotions – frustration when solving a case took longer than he liked, amusement when Yako fell into one of his obvious traps, satisfaction after a nice meal (but never too satisfied, because it was never quite enough). Human emotions came about for entirely illogical reasons, like Yako crying after they had finished their most recent case. She had no connection to the people involved, and he for one thought she should be satisfied that it was solved, especially given that she had gotten some food out of it herself.

He thought back to the image of her with her back to him, fist trembling in anger.

Right. She had been angry at him.

Why?

Neuro hated that question. He never really had to deal with it back in the demon world. Reasons for mysteries were simple enough; it was the how that had always been the interesting and tasty part. On the surface world, though, the why contributed to his meals much more. Yako was right about that. They were the flavoring that made them so much more delicious than the ones he knew back home.

There had been a why to the conversation last night that had escaped him. There was an underlying component, something she hadn't said but meant for him to hear.

"Louse, I don't understand what you don't say," he grumbled in a fit of frustration. A useless action given that she couldn't hear him. His stomach responded with equal frustration. Hunger was not making this effort any easier.

Deciding to take a different angle, he scooped up the remote for the television and turned it on. It was about time for Slave number two's favorite time-wasting activity to make its appearance, and there was much anger and crying in that show. Perhaps he could draw some conclusions by observing it.

...

Yako did not appear the next day either.

This time, reluctantly, Neuro sent Evil Friday to the task of searching her out. He needed to make sure nothing had happened to his cover. After all, starting over was not an idea he took any pleasure in. Whatever concern he might have had that he would have to do just that was banished when he found her simply going about her day, though he left the demonic eyeball to follow her nonetheless.

Observing the show had done little to assist him. He could name the flavors of emotions but could not understand the reasons for them beyond the simplest expressions. Characters cried when they were hurt physically; he understood that, because pain brought about tears, but then they also cried when they were not hurt, and they cried when they laughed too hard, and he began to think crying was not tied to pain for humans.

He found his fingers drumming the desk he sat on. His mind was drawing a blank.

There were aspects of him that were conforming to the common sense of the surface world. His physical ability, for instance, was declining, and the power of the 777 Demon Tools likewise. His healing rate had decreased, sensations of pain had thus increased, and he rested a bit more often.

What was not changing was his brain. Its functions remained as they always had, and his thought process and ability to comprehend had not changed. The one part that might be of some assistance to understanding Yako's underlying message was not at all becoming human. He was grateful at least something about him wasn't deteriorating, but mildly annoyed all the same.

His fingers continued to drum on the desk.

Movement out of the corner of his eye drew his gaze to the braid of hair sticking out from the wall. Slave number two had been in a mood when he came by and didn't find Yako (and was promptly booted for asking too many useless questions), and the mood had somehow rubbed off on the other resident of the office. Things had been quiet after some initial flailing in frustration, but now the animated-hair-turned-secretary waved his attention to the monitor she had turned to him.

On the screen, she had typed Need help yet?

He gave his secretary a sidelong glance until a frown consumed his features. "No," he replied firmly, rising to move around the desk where he sat purposefully in his chair. The hair typed in a set of characters that together looked like an annoyed frown. He ignored it.

On his own computer, he opened a blank word document. Then he blanked. His tapping resumed until he realized he had started again, and he moved his hands across the keyboard.

Obviously this mystery required organizing, so he put it down on the electronic paper.

Yako is angry. Why?

He still hated the why.

Conversation about emotions. They flavor mysteries. They were not important, but now have reason to be.

The cursor blinked as he paused on that. Then he went back and underlined emotions because obviously this was the important part of the topic. Emotions were now of more importance to him than they were before because he now understood how they affected his meals, but that didn't mean he could suddenly understand them. That aspect was still lost to him.

Maybe…

I don't understand human emotions.

"Maybe it's about time you did consider them." Yako's voice reminded him.

He doubled back on that, replacing it with I don't consider human emotions. Then he thought about it.

Yako is angry because I don't consider human emotions.

Fantastic.

But why?

...

Day three of Yako's disappearance.

She hadn't actually disappeared. Evil Friday was still relaying images to him. At one point she even hesitated at the train station, apparently unable to decide whether to go home or to the office for all of five minutes.

Neuro had yet to figure out why his being inconsiderate of human emotions gave Yako a reason to be angry. He was also well aware that this was a useless waste of energy, and though Yako had left him a mystery to solve, it would be one he would be unable to consume.

His conclusion yesterday answered other, less important whys about the situation. Why wasn't Yako coming to the office? Because she was angry with him. If he had learned one thing from that dreadful show it was that when humans were angry, they avoided the object of their anger. Why was Godai in a mood? Because Yako wasn't in the office. Why was Akane insisting he let her help him? Because Yako was not coming back to the office.

Why was he not accepting her help? Because Yako was angry with him, and he was frustrated by his inability to comprehend why.

A part of him praised his newfound ability to deal with certain whys. Another part of him was a persistent reminder that those kind of whys were merely surface questions. Slave number two's mood change was because of Slave number one's absence, but why her absence caused his sour mood was not a question he could answer.

Why was it that Yako was angry that he would not consider human emotions?

He tilted his head back over the arm of the couch too short for his stature. Perhaps the complexity of human emotions was the ultimate mystery he was looking for. He frowned, repulsed by the thought.

"Damn it, monster, I'm getting sick of this place," Godai snapped from his desk, loudly dropping the pack of papers he had been ordered to go through. "Go apologize to the girl for whatever you did to upset her."

He gave the man a long look that eventually left him squirming and looking for an exit. It brought a momentary smile to his face.

Slave number two threw up his hands in a gesture of surrender as he moved toward the door. "It's a suggestion, a suggestion! Not an order. I'm gonna… you know, go get a stock of bento for the kid. Pass out fliers… check on the company… that stuff." And with that, he quickly slipped out. Neuro watched the door as he listened to the hastened footsteps of a desperate retreat. Humans could be affected by the smallest of things sometimes.

Huh.

Something clicked. His eyes widened on the door, upside-down from his current position.

He had been thinking about this whole thing from the wrong angle, he realized suddenly, and bolted upright. He had been so focused on the flavoring, on the emotions, he forgot entirely about the human half of it. It wasn't that he wasn't considering human emotions, it was that he wasn't considering that they affect humans.

Yako was angry because he was not considering that emotions affect humans. Why? Because she was affected by human emotions, her own and possibly others. It explained why she cried after their last case, as others involved were crying, and if humans were affected by the emotions of other humans, then it made perfect sense. He should have learned that from Aya's case.

Of course he wouldn't understand them. He couldn't begin to. Yako knew that, and rather than try to explain to him the intricacies of each feeling, she had linked them with the one thing they had most in common – their love of food – in order to express their importance. Not just their importance to him, but their importance to her.

He would have to reprimand her for having not just come right out with that. That would have…

Wait, no, she had. Right before she had walked out.

"Akane," he called to his secretary as he stood up from the couch. "Call for Yako. I believe I understand now."

Nothing happened for a moment longer than he thought it should take, which drew his gaze over to the braid in the wall. She flailed before typing up a response.

You need to go to her.

"Nonsense," he barked. "Why should I go to her?"

He frowned. There was a why again.

Akane snapped against the wall in what appeared to be frustration. You wouldn't understand, she wrote quickly. She's waiting for you. Just go to her.

He wouldn't understand? No, of course not. It had to do with those complicated human emotions Yako was angry he wasn't considering. Relenting, he pulled out his phone himself.

Snap, snap!

Unamused this time, he looked back at Akane.

In person. It's better.

"I see no reason simply mailing her myself is any problem."

Just trust me, she insisted, with the addition of an angry face created by characters.

"And if I don't go to her?"

Starve.

He tapped his finger against his leg. That was a real possibility. He had not been feeling his best to begin with, and the addition of three days with no meals while he expended the energy to figure out this whole mess and also keep Evil Friday on her had added to that. Part of the reason he had been lying on the couch was for rest, though he was too entrenched in his thoughts to actually sleep.

For a demonically-enhanced braid of hair, Akane was sharp.

He stood then, resigning himself to the task. Perhaps fetching her himself would also get him a meal faster. He tuned into the image Evil Friday was relaying back to him, though finding the image to be of Yako's angry face surprised him, and he winced as the eye was suddenly impacted by a fast-moving hand. The tool and image vanished.

He blinked with wide eyes. Yako was still angry.

"Akane," he finally started. "I require some advice."

...

The last image he had been relayed by Evil Friday did give him her location, but by the time he had reached the school, Yako was nowhere to be seen. A wild chase was not on his to-do list, nor did he feel he had the energy for it, so he simply turned in the opposite direction and went straight to her house. Her housekeeper was just leaving for groceries as he approached the front door, and quickly recognized and informed him that no one was home at the moment, though invited him to stay. Apparently Yako would be returning from school shortly. She said nothing about her current absence from the office, so he guessed she had never discussed with her household the conversation that had left her so angry.

Alone in the living room, Neuro took to observing. An alter was set up in the corner of the room, and he recognized the face of her father in the picture when he approached it. She had cried then, too. He didn't remember what the others in the household were doing at the time, but she had cried before, during, and after the case. He wondered vaguely the reasons for her tears.

He heard the front door click open.

"I'm home," Yako called out. Her tone lacked happiness, likely as a result of finding his tool following her if her reaction to it was anything to go by. Footsteps sounded behind him, and he turned to her when he heard her gasp.

"Neuro…"

Akane had coached him on what he should say to get her to come back. Now, if only he could remember how he was supposed to start…

"What are you doing here?"

"Retrieving you," he answered. This was as good a place as any to begin this conversation, though he wished she wasn't immediately angry. She had meant to avoid him, because he was the object of her anger, but Akane had insisted she was actually waiting on him to move first. It didn't seem like Akane was right at the moment, but perhaps he needed to be patient.

"Why?"

"You thought you could simply abandon your role for three days without someone coming to bring you back, louse?"

"What, were you worried? So much you had to spy on me? I thought you didn't understand human emotions."

"I don't."

That had been too short a response, he realized, as Yako quickly turned heel and fled for the door.

"Ya-" but he was cut off by two things, the first being the sound of the front door slamming, and the second being his sudden bout of dizziness. He grabbed for the couch, missed completely, and found the floor instead. His energy was spent.

It took him a few moments to collect his bearings, and when he finally did he moved to lean back against the side of the couch. A meal and rest were required if he wanted to get back to normal functions. A meal and rest and Yako, who was currently running away again.

Neuro retrieved his phone from his pocket. Her name and address were quickly brought to the screen.

Come back, Yako.

After a few minutes with no response, he sent another.

You are needed.

He waited.

I am too weak to chase you, he added when time ticked by some more.

Unwilling to say anything more through mail exchange, he tipped his head back and closed his eyes, hoping she would simply return without the need for more effort on his part. He didn't have any to expend right now.

"…Neuro?"

His eyes snapped open, and Yako was standing next to him. Had he drifted off? He hadn't heard her return.

She kneeled down next to him, her expression no longer one of anger but of concern.

He remembered something like that look from when he had collapsed during the HAL case. Yako wasn't in danger, yet she grew concerned because… because he was weakened. Huh. It appeared human emotions were influenced less by the emotions and more by the state of others around them. If someone was in a state of unhappiness, others around them may be unhappy, not unlike Slave number two and his secretary back in the office. Criminals often had quite opposite reactions, which seemed abnormal compared to the status quo.

Perhaps that was what angered Yako so much, he realized. His responses aligned more with criminals than with the others involved.

"I am in need of a meal," he explained, sitting up a little straighter in an effort to appear less exhausted. Her own posture relaxed a little, at least until she caught his gaze. They said nothing for several moments. Yako looked away.

"I spent the last few days trying to do something I normally do not," he began to explain. Akane had told him to detail to her his efforts to solve this particular mystery, and while he did not think it necessary, he also did not think he knew best on that subject. "I tried to answer a why. Specifically, why you were angry.

"I do not handle the whys of a case. They are usually too linked to human emotions for me to even begin to understand. I did not understand the reason for your anger the other night, however obvious it might have been to you."

Her eyes came back up to meet his, surprised, before turning away again.

"Sorry, Neuro."

He hadn't expected the apology, and it took him a moment to collect his thoughts again. He could end the conversation here, but…

"I believe I may have solved that mystery, though." Why put the effort to waste by not explaining it? Especially when Yako's expression was enough to make him grin.

"Wait, really?"

"You were angry because my inability to comprehend human emotions was also causing me to neglect their importance to you and how they affected you," he explained. "You react to the state of others in a way you understand to be appropriate, but I do not. Because I have never considered human emotions, I also never considered that they have such a great impact on humans. You were upset after the last case because others were as well, and I pressed along without caring."

He paused, uncertain suddenly because her eyes were brimming with tears. Tears were an indication of pain, he had thought, but his other slave's favorite show had shown him otherwise. He took a breath and leveled her with a serious expression.

"I cannot understand human emotions, which is why I need you. You can unravel them where I cannot. I can answer the how while you take care of the why. Whys are far too complex for me to make sense of when they are tangled up with human emotions. This is not true for you. I will never understand, but I can attempt to consider them and how they affect you. Understood?"

Now she was crying, but her mouth was pulled into a smile despite the tears. She quickly rubbed at them as she nodded.

"You're crying now, but the situation is different from before," he noted. "Explain."

She sniffled. "I'm really, really happy," she answered. "People cry because of an overwhelming feeling. Sometimes that's sadness, and sometimes it comes from being very happy. Right now, I'm overwhelmed with happiness."

"That's contradictory."

She laughed, "Try to think of tears less as being linked to an emotion, and more as being linked to being overwhelmed with something. You might be able to tell which end of the spectrum its coming from if the person is smiling or not."

He hummed in consideration. Overwhelmed equals tears. Mouth could indicate emotion connected to it. He filed those away for future reference.

"Thank you, Neuro."

He was struck by the brilliant smile she suddenly presented to him, eyes still glossy with tears. She didn't even smile at a free meal like that, and he doubted she was confusing him for food. There were few times when she left him speechless, and even fewer when she left him thoughtless. It appeared this would be one of those times when he could find nothing he could say in response.

"Hang on just a moment," she said suddenly, standing quickly and taking off to a separate part of the house. He craned his neck in curiosity to look over the couch after her. She returned shortly with a box in her hands.

Upon opening the box and dumping its contents onto the floor, she looked up at him with something of a sheepish expression.

"Picture puzzles aren't really the kind of puzzles you like to eat, but…"

She was trying to feed him with this? He scoffed, though his smile wouldn't leave his face.

"This is hardly a crumb, louse," he noted, picking up a piece to look it over. "I doubt this would satisfy me for more than a few minutes."

"Think it'll give you enough energy to get back to the office, at least?"

He snapped two pieces together.

"Perhaps."

...

In the end, Yako ended up with pieces in her nose. Neuro felt much better after that.

It didn't quite feel right until I wrote that last part. Reviews are positively wonderful. Favorites are cool, too.