This series is the result of a theory on tumblr that I took way too seriously (there's a link to it on the AO3 version of this) about the idea that Akabane Karma was raised by Neuro and Yako and isn't entirely human. However, because I feel strongly that Neuro and Yako's relationship is platonic, I wanted to write a group of stories about the three of them with the twist that Karma was adopted rather than their biological child. This is the prequel story about how the detective duo ended up adopting Karma as a baby.

In the interest of full disclosure, there aren't many Assassination Classroom characters in this first story because they're all very young at this point. I plan to include more in future stories. Also, I know basically nothing about babies, so I'm saying that anything I get wrong is due to differences between human and half-human/half-demon babies.


"I smell a mystery, slug."

After half a year of getting used to his sudden appearances again, Yako didn't even jump. "C'mon, Neuro, I'm eating. I'm supposed to meet up with Kimura-san in a couple minutes."

"This one is strange," Neuro commented, casually pulling her into a headlock. "It feels like there are demons involved."

Yako struggled out of habit rather than any real thought that she could get away. "Other demons? I know there's you and Zera, but I thought the human world was poisonous to de– oh, that's why it's strange, isn't it?"

Ignoring her, Neuro continued: "The scent of demonic activity doesn't linger long on Earth. Come immediately and I'll use your head as the test subject for Evil Hammer."

"I think you mean 'or' you'll use my head as a test subject." Yako continued to struggle as Neuro dragged her out the door. "I'm coming, okay? Give me a minute to pay the bill."

"No time."

This must be something big, then. Yako sighed and made a note to go back to the café later. She knew the owner pretty well, and it wouldn't be the first time she'd left Takashima's in a hurry for a case, but it was still rude. "Let me at least text Kimura-san." Kimura Ran was a police officer that she'd met during a negotiation mission last October. Kimura was about ten years older than Yako, give or take, but she was so enthusiastic about her work and loved cooking almost as much as Yako loved eating, so they got along splendidly. Their friendship had been cemented after Yako caught the criminal trying to frame Kimura for murder. The case was enough to get Yako's foot back in world of detective work, and when Neuro returned from Hell her career shifted even further back into investigative jobs.

Neuro dropped her on the ground and kept walking, intent on the mystery. Yako knew from experience that if she lagged too far behind, she'd probably end up with her shoulder dislocated or wallet missing. She hastily pulled out her phone and Akane, currently in her mobile form, offered to compose and send a text to Kimura.

It was sunny when they left the café, but by the time they got to the public park about ten blocks east the sky was starting to cloud over. The park was eerily empty, perhaps because of the muggy weather and the hint of coming rain. There were no old ladies on the benches talking about their grandchildren, no cyclists swerving to avoid soccer balls and wayward toddlers, and no musicians starting up sets on the grass.

Then she heard the crying.

Neuro was about ten meters in front of her and there was another screeching sob as he approached a bench next to the pond. Yako knew the feeling. She jogged to catch up as the demon bent over a small, abandoned baby carrier perched on the bench. "What's a baby doing out here all alone?" she wondered, looking around. "Is there someone else here?"

She peered in the carrier and blinked: there was no child. Neuro was quicker than her and identified the source of the sound crawling around just off to the right in the dirt underneath an oak tree. Before the demon could do something regrettable like twist the kid around or break a bone, Yako snatched the child up from the ground, holding it close and getting a better look.

Yako didn't know much about babies, but this one definitely looked like a boy, with wet, red hair falling in his chubby, inquisitive face. He was old enough to be crawling around, maybe close to walking? His eyes were a curious gold color, and there were red streaks down his chin and on his clothes that didn't look like fruit juice. "That's…"

"Not human," Neuro finished, leaning in close. His own eyes were glowing at the puzzle in Yako's arms. "This one feels like a demon, as I thought, but… muted. He's either a very low-level demon that has adapted to thrive on Earth or the result of an ill-fated encounter between a demon and a human. Given the conditions we've found him in," he added, gesturing to the discarded carrier, "I would guess the latter. Children are hard to come by in Hell and none of mine would knowingly leave their offspring here unattended, even those of lower power."

Yako was a little touched at the sliver of morality in the monsters until Neuro continued, "If they really didn't want the brat, he'd be fed to the Emperor's hunting dogs instead." Yako exhaled and squeezed the child a little tighter. He'd stopped crying the moment Neuro got close, which was probably a better indication than the unsettling eyes that he was a demon. Otherwise, it meant the kid had an extremely poor ability to judge danger. It would probably get him into trouble someday.

"Well," Yako began, "I haven't had any work involving missing children before, but I guess it would make the most sense to take him to child services or the police until we've tracked down the human parent. We can reunite them."

"You've missed something." Neuro pointed at the boy's clothes, which were wet like his hair. "It hasn't rained today."

"What? Then why…?" Yako looked back at the carrier, and then past it to the pond. An ugly anger filled her stomach as she realized what happened. "Oh my god. Someone tried to –" She broke off, ready to punch something and glad she was holding the little boy so she couldn't. "Neuro, we're going to find this one. Human, demon, whatever did it." The blood streaks made sense now: someone had probably held him underwater and he'd bitten them with those sharper-than-human teeth poking out.

"We will solve the mystery as always."

"Any thoughts on where to start?"

She expected the sharp, mildly terrifying smile that came out during cases when no one else was looking, and she wasn't disappointed. "Evil Retrace will lead us to where the brat basket came from."

"You mean the baby carrier?"

"That's what I said. Evil Retrace only works on things that aren't alive, so it would be pointless to try tracing the imp back to wherever he was before."

"If we're tracing the carrier, then wouldn't it just lead back to where it was manufactured?"

"Fool. Evil Retrace can see the path of an object back two hours in time, no more." The air around Neuro shifted, the way it always did when he revealed his true form, and suddenly there was a grotesque creature in his hands, the ugliest mouse Yako had ever seen. There were too many eyes and unlikely limb proportions and stinking ooze and claws that were frighteningly long.

The Retrace monster scampered in circles around the carrier for a minute before darting off towards the park entrance. Neuro was after it in a flash but Yako took a moment to grab the carrier and buckle in the baby before taking off after them. The kid wasn't fazed by the jerky movement, gnawing on a rubber toy in silence.

"Neuro, slow down!" The demon pretended not to hear her and eventually Yako had to stop, setting the carrier on the sidewalk and putting her hands on her knees while she caught her breath. She'd already been sweating buckets from the humidity, but running and missing lunch took their toll too. Rain would be nice. Then she laughed at the unexpected thought, still panting. She'd only just gotten back last week from a three-month case in India, where monsoon season was in full swing, and here she was wishing for rain already.

India had been beautiful, breathtakingly so, and Yako ached to go back someday. During the case she'd met some wonderful people that she was sorry to leave behind, and the curry there was to die for. A rash of thefts had left the country missing twenty-three of their most prominent national treasures and Yako had been called in to solve the case. It had been hard because the twenty-three sites were scattered across the subcontinent and all unaffiliated with one another, but she and Neuro had split up for the case to cover more ground and move the investigation along faster. It was further proof that he trusted her detection skills now, despite his verbal abuse over the phone whenever she called to check in with him. It was satisfying to tie the case up as an equal with him.

Yako was jerked out of her memories by the baby, who started acting up again. "Hush, shhh, it's okay. Are you hungry?" She held out the orange that had been stuffed in her purse, suddenly aware that she had no idea what to feed a (possibly?) one-year-old half-demon. The boy shrieked louder and people on the street were starting to stare. Yako could just picture the headlines now:

"Former High School Detective Secretly a Parent?"

"Child Cries at the Piggish Negotiator née Detective!"

"Katsuragi Yako: Kidnapper?"

Maybe he'd calm down if she picked him up? Yako reached down, intending to release the buckles, but the boy got a hold of her finger and bit down with teeth that felt as sharp as they looked. "OW!" She withdrew her hand, wincing at the marks on her thumb, and the baby giggled. Then she felt claws clamp onto her shoulder and pull her into a pretzel. "OW! Neuro!" The demon detective was the culprit the second time, and Yako shrugged out from under him when he relaxed his grip, her muscles aching from the strange angle they'd been forced to accommodate.

"The mystery isn't going to wait all day," he hissed, then straightened up and projected his voice to the listening strangers. "Sensei, I know you like it when your boyfriends bite you, but you shouldn't make an infant satisfy your masochistic tendencies."

"Neuro!"

"Come, Sensei, your humble assistant has retraced the necessary steps and it's time for us to visit the police station."

"Oh." Yako gripped the baby carrier's handle, careful to keep her hands out of reach of the child. "I guess we should get to that, then."

"As you wish!" Neuro trilled, his innocent façade shining with pure enthusiasm. Yako rolled her eyes.

They walked for a few minutes before Yako began, "So whoever was carrying him came from the police station?"

"Evil Retrace ended in the middle of a street when the time limit was reached. The imp was in the park for over an hour."

"Okay," Yako replied, deciding to play along with his vague explanation, "so why are we going to police station? I'm guessing you're not really interested in the baby's welfare, so this stop probably isn't to hand him off to them."

"This child is our only clue to a mystery that is certain to have a strange flavor. I wonder if a mystery created by a demon in the human world will taste different."

That still hadn't answered the question, but Yako connected the dots herself. "We're going to look for any cases that look like the work of demons because we know that the baby'd nonhuman parent was in the area at one point?" Neuro frequently stressed the importance of blending in with humans – which Yako personally thought was hypocritical, given all of his over-the-top antics during cases – but it wasn't impossible that other demons would be more lax in their camouflage. "Why not ask Godai if he's heard of anything?"

He waved her off. "Slave #2 is working on another project for me right now, one of higher importance than this."

"He's Slave #2? C'mon, aren't we past this? I'm not a slave anymore! Am I?" Neuro gave a self-satisfied smile but didn't answer. "Am I?"

"Ah, look, the police station." Neuro waved at the officers, chattering about what "Sensei" was up to lately, and Yako dropped the subject. He was probably teasing her, but he'd been in a fey mood since they got back from India. Of all the foreign countries they'd travelled to for cases, he had seemed to like it there the best. Because they'd been split up for a while, Yako didn't know what he'd seen that sparked his interest in the place, but hints in their conversation suggested that he genuinely enjoyed the country. He'd been dogged about learning to do henna, which was a surprise – he never showed interest in anything that couldn't bring him a mystery – until Yako realized that he wanted to draw it on her. Thankfully, the dye was almost invisible now – Neuro favored obscenities over flowering designs, and she'd had quite a time getting taken seriously despite her reputation.

Yako eventually waved over Ishigaki and Todoroki and explained that she and Neuro were looking for reports of especially strange crimes. The baby carrier got raised eyebrows from Todoroki, but Yako jumped in with a quick, non-demonic description of events before Neuro could make up something embarrassing.

Todoroki pointed Neuro in the direction of the records room and pulled Yako aside. "You can't cart around a baby during your investigations, Yako. For one, it's dangerous. Two, you could get in some serious trouble if someone reports him missing and you have him. He should be with child services."

"Okay," Yako replied weakly. Neuro's not going to be happy, but it's not like there's anything we can –

BANG! Yako immediately ducked and Todoroki whipped around, hand on her holster. There was smoke pouring out from the vending machine next to the restroom, filling the room.

"Oh dear," she heard. "I was just trying to get Sensei a green tea. I don't know what happened." The smoke detector tripped and the alarm went off, water spraying violently from the overhead sprinklers.

There were groans around the office as paperwork got soaked, but everyone moved briskly towards the exits. Yako was ready to join them with the baby, but Neuro gripped her arm and dragged her further into the station.

"What are you doing?"

"We need the brat until we've found our case. Let's go." So it was a distraction, then. They ended up in the (thankfully dry) records room with the baby carrier on a desk while Evil Fridays crawled over the cabinets that held case files from the past few months.

Yako picked up a file at random and began skimming the first pages. "You're going to blame me for this anyway if we get caught," she explained when Neuro looked at her askance. "I might as well do something."

Twenty-eight minutes later, the Evil Fridays began to squeak. Yako was not entirely sure how they did this, given that they were semi-mechanical eyeballs with no visible mouths, but she'd recognized that there was no point in questioning the 777 Tools anymore. "Did they find something?" She plucked one wayward Friday out of the baby carrier before the kid could try to eat it and moved over to the section of floor where more Fridays had migrated excitedly.

Neuro picked up the file and examined it more closely. "Indeed, it's a double homicide on the other side of Tokyo that was especially violent. Almost two months ago. No suspects arrested." Yako peered over his shoulder at the crime scene photos of Kanemoto Denji and Fujino Hideaki, who'd been found completely decimated in a less-travelled alley. It was… grisly, so much so that there'd initially been some speculation that a wild animal had caused their injuries. There'd been two knives found around the bodies, possibly indicating that they had attempted to defend themselves from whoever or whatever did this to them. However, the knives were clean and none of the blood samples collected at the scene could be linked to a third individual. It had been tentatively ruled as yakuza work because of the area where the bodies had been found – the police thought that the victims may have inadvertently witnessed an illegal transaction.

"You think this was a demon?"

"The level of carnage is right. They were ripped apart without any human weapons."

Neither victim had any living family members, but the two had worked in the same office building, so Yako, Neuro, and the nameless child took the train over. The ladies at the front desk immediately recognized the famous Katsuragi Yako and her assistant, and they were ushered to the floor where Kanemoto and Fujino had worked.

Yako approached the man at the desk next to Fujino's former workplace. "Excuse me, I'm sorry for bothering you, but could you tell me a bit about Fujino-san and Kanemoto-san? Was anybody angry at them, that you know of?"

"They were bullies, the pair of them," the man answered bluntly. "If there was any interpersonal trouble in the office, it was caused by those two. They tried to haze the newcomers and were consistently crude to the women in Marketing. I don't reckon anyone here killed them, but I can say that no one is sorry that they're not around the office anymore."

"Oh," Yako replied, surprised by his honesty. People tended to speak better of the dead, especially if they thought they were being suspected of murder.

"They've had reports of harassment filed with the police against them, from what I've heard, but if you have access to their files, detective, then you probably already knew that. Never formally charged, because there wasn't enough evidence for a court case, but…" he gestured vaguely. "It's my opinion that they did it."

Yako thanked him for his help and talked to a few other office workers, who shared the same opinion of Kanemoto and Fujino. She made a note to take a closer look at the victims' personal files, which Neuro had skimmed in the police station but hadn't discussed with her yet.

When the child started to kick up a fuss again, Yako beat a hasty retreat to a nearby convenience store to buy diapers and eight boxes of the tasty-looking snacks that were on sale at the front. Changing a baby couldn't be that hard, right?

Yako struggled in the ladies' room to get the diaper folded right. Akane fretted from her spot on Yako's phone, but didn't have any useful experience either, so the detective ended up calling Kimura-san, desperate for advice. The older woman talked her through it over the phone, very kind but also amused and curious about the kid. And on that note, Yako realized that she couldn't just call him "the kid" indefinitely. "How old is he, Yako-chan?"

Yako groaned as the child's screech threatened to deafen everyone in the convenience store. "Dunno. More than a year, I think? How did you get Masayoshi so well-behaved?" She'd seen Kimura's baby boy before, and he was a sweet, quiet slip of a child.

"Well, Yoshirou and I both come from big families, so we've had practice with kids. We've only got the one right now, but since he's fourteen months old we're thinking about more."

Yako smiled. "Good luck, then. Thanks for helping," she added genuinely as she wrestled the kid back into the baby carrier. She stifled a gasp when she saw that his grip on the changing board had left a hand-shaped dent. He was strong.

"Anytime. Feel free to email me with any more questions until you get him back where he belongs, okay? And make sure to feed him. If he's a year old then he's probably okay to have the same foods as Masayoshi. I'll send you our list."

"Got it. Thanks again." After opening the list on her phone, Yako bought more food and took the baby to a bench outside, peeling off the top to a yogurt and stirring in the mashed strawberries. Mmm, the yogurt was pretty tasty. "Wait, no, no, I have other food, this is for him. Sorry, little guy." She shook her head to clear it and pulled out a new spoon. "Open up." The now much-quieter baby opened his mouth obediently and, aside from chomping down on the spoon with more ferocity than expected of a one-year-old, his behavior seemed to suggest that he was content.

She looked up to see Neuro standing in front of her, with no indication of how he'd gotten there. "Anything else from their office?" she asked.

"No."

"One of them was saying something about the victims being accused of harassment. Did their files have the names of the women who accused them?"

"Yes. It only took some quick computer searching –" via Evil Script, probably "– to get their home and work addresses as well. We will take Slave #3 with us."

"He's got a name."

"True, but, as we are not in possession of it, 'Slave #3' is functional."

Yako opened her mouth to argue, then realized that the probable alternative was Neuro making up a name for the kid.

Not the best idea. Calling him "Three" would work until they knew more