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Published:
2024-09-04
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2026-06-06
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12/?
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Spending Spring Break Underground

Summary:

Once upon a time many centuries ago, the world was ruled by two species: Humans and Monsters. Both of these creatures thrived together using their special talents to help one another. That was until the humans declared war on the monsters out of the blue. This led to the monsters to be banished beneath a mountain.

But rest well when I say the humans suffered their karma. Without monsters they soon lost their magic, along with their way of life. They developed their own powers called quirks, with this power came the raise of heroes and villains. Quirks were quick to spread until only 20% lacked them.

It wasn’t long before magic and monsters were reduced to legend. This led many to climb the mountain to see if the legends were true, only to never return. This was because of the heartbroken monster king who put a bounty on every human who fell.

When the last human fell, the monsters were ecstatic. Finally they’ll be free to wander the surface to feel the sun and the rain again! But unbeknownst to them, they have become even more trapped than they had before. With each reset the monsters were forced to live the same day over and over without anyone knowing ... .Well, almost everyone.

Notes:

I haven’t found a Undertale x My Hero Academia fanfic that scratched my brain in the right way, so I made it myself. If you don’t like it……why the fuck are you here for? Get the fuck out. To everyone who’s not a miserable jerk, I hope you enjoy.

Chapter 1: Two Broken Souls

Summary:

Sans woke up after another reset. He has finally gotten used to the dismay at the start of each run. So of course the universe had screwed him over again, but maybe it won’t be so bad.

Chapter Text

Sans sighed as he sat at his post. The splintered wood felt rough against his bones even with his hoodie on. Why was he even here? The same things are just gonna happen: The human will come through the door, play around with them for a little bit before moving onto Waterfall. Whether Papyrus survived was completely up to the them. Because no matter how much Sans reasoned, begged, or threatened his brother always confronted them. And at that point all he could do was pray that the little demon was feeling merciful. Heh and here he thought the flower was sadistic, this “kid” was on a whole new level. Maybe he should just stay home this time, maybe he can-

{CREEEEEAAAAAKKkkk….}

Sans flinched as he snapped his head towards the Ruin doors, the doors that shouldn’t be opened yet. It was way too early for the kid to enter Snowdin, what did they do? Did they kill Toriel already? Is this their way of shaking things up?

Sans felt his hands grip onto the frayed wood of his station. Fine, two can play at that game. If this kid wants to take it up a notch then he’ll give them the full course. He’ll forgo the whoopie cushion and go with a more direct approach. The kid was getting better after all, might as well get some kills while he still can…Man he’s messed up.

The skeleton shook away his thoughts as he teleported to his regular spot. He turned his head expecting to see the familiar sight of a brown-haired human marching their way down the snowy path, only to do a major double take when he instead saw a bush of green and black hair. That bush was connected to a baby face that was dotted with freckles and sporting two big, green doe eyes. They were wearing a pure white sweater with a forest green vest, navy blue shorts, and bright red sneakers. But the most shocking thing of all was how small they were. Now Frisk was by no means a tall kid but this one could barely reach up his knee! They couldn’t be older than nine, what was a little kid doing here?

Scratched that, why were they here period?! There was never a reset where he even saw a glimpse of this kid, let alone seen them. Did Frisk put them up to this? Did they decide to add someone new this time? Do they know what’s going on or are they ignorant to the crimes that were committed? The kid looked innocent enough but so did Frisk and look at what they did. Stars, why can’t he catch a break?

The skeleton sighed as he rubbed his nasal rig, well he might as well CHECK them, he’s gotta know what he’s dealing with. Sans eye felt warm as he did a quick check on the new human, and what he saw shocked him.

[NAME: Izuku AGE:8 PRONOUNS: HE/HIM

DEF: 15 ATK:20 SOUL TRAIT: HEROIC

He’s amazed by the nature around him.]

Oh god this kid was an even bigger anomaly than the last one! For starters the kid has surprisingly good states for his age even though he didn’t have a hint of EXP to his name. But the weirdest thing had to be his soul trait, a trait he never seen before. But he swore he heard it somewhere before, but where could he-

“Hey are you okay?”

Sans felt his soul jump at the sound of the tiny voice breaking through his thoughts. His head snapped to the kid, Izuku, who flinched at his reaction. The kid nervously pulled at his fingers as he shrunk into himself, “I-I’m sorry! I didn’t mean to startle you. I just…well you looked…sorry…”

Izuku hunched into himself even more as Sans stood there dumbfounded. That was not what he expected, he didn’t know what to expect but it definitely wasn’t that.

It was the polar opposite of Frisk’s reaction to when they first met. They were on edge and a bit annoyed but they seemed to get used to him quickly, a bit too quickly if you ask him. But this kid was acting like he was gonna get hit just for spooking someone. And this wasn’t an act either, the kid was truly scared right now.

This brought an odd feeling to his chest that was quickly becoming uncomfortable, it was like someone wrapped heavy chains around his soul then dunked it under water. He had no idea what this feeling was but he needed to put an end to it, and quick.

“hey don’t sweat it kid, you just caught me by surprise. normally people don’t just walk up to random strangers in the woods,” Sans said with a laid back grin, the kid didn’t seem to believe him though.

“I know, it’s just that you looked really nervous and stressed and I wanted to see if you were okay,” The kid finally looked him in the eye sockets, his eyes were full of worry. And isn’t that terrifying, having his laid back ruse being figured out by a little kid? It took everything in him not to break his composer.

“Eh that’s what happens when you become an adult kid, but I think we started on the wrong foot,” He held out a hand to the kid, he ignored how the little guy flinched, “don’t cha know how to greet a new pal? shake my hand.”

Izuku just looked at his hand for a bit before he finally got the courage to grab it. Hesitation quickly turned to surprise when the hidden whoopie cushion let out a loud “PFFFFFFfffffftttttt”. Sans smiled as he pulled his hand away and lifted his glove to reveal said whoopie cushion, “the ol’ whoopie cushion in the hand trick, always funny.”

It took a minute for the kid to fully register what happened, but when he did he quickly dissolved into a fit of giggles. Against his better judgment Sans felt himself relax a little at the appropriate response. The kid was just acting like, well, a kid. Like a genuine kid, none of that wrongness he felt from Frisk. Which was a relief all on its own.

“heh i’m glad you have a good taste in comedy,” He lazily stuffed his hands into his pockets, “ the names sans, sans the skeleton, you’re a human right?”

The kid nodded as he smiled brightly, very brightly, “Mhm, my name is Izuku Midoriya, it’s a pleasure to meet you Sans!”

Sans shrugged,“eh i wouldn’t say that, technically i’m supposed to capture humans but i’m too lazy to do it. My brother on the other hand is a human-hunting fanatic and would captur you on sight.”

Izuku tensed a little at that, “Right, because you need human souls to break the barrier right?”

Sans blinked in surprise, he thought most knowledge about the war had been discarded on the surface, “yeah your right, but what kinda monster would i be if i let someone with such great taste in humor to get captured like that?”

It was the kid’s turn to look surprised, “......You’re going to help me?” His voice was hopeful yet cautious. The kid obviously wanted the help but he didn’t believe he was going to get it, that odd feeling was back.

“yeah, I’m not exactly hoping to sit back as a kid walks around with an unearned bounty on his head. i may be a monster but I’m not that kinda monster, heh.”

The kid seemed to think over his words carefully, he pulled at his fingers so hard Sans was afraid he’ll break skin. The kid looked at him, then towards the Ruins then back towards him before he finally said, “Okay…Thank you..,” he gave him a small yet genuine smile.

Sans smiled back as he made his way out of the tree line, “Don’t sweat it kid, just try to relax okay? I’ll keep an eye socket out for ya,” Sans looked behind himself, the kid was following behind him cautiously, still looking tense yet hopeful. They kept walking in comfortable silence until they reached the infamous bridge.

“Hey you see those big poles over there?” Sans pointed at said objects, Izuku nodded, “It was made to stop people from getting through, but it's spaced out so wide anyone can cross it.”

To prove his point, Sans easily walked past the poles, Izuku following close behind him. His green eyes scanning his surroundings in wonder only a kid could have.

There were a few times where Sans had to stop the kid from getting a face full of snow with his magic. Any embarrassment the kid had would always be replaced by amazement from it. It was hilarious to say the least, and made the tense and boring trip a little more enjoyable.

It wasn’t long until they passed by his post, usually that’s where the kid would first meet Papyrus, but he won’t be here until much later. Damn this kid has only been here for less than an hour and he’s already doing everything differently. Who knows maybe they won’t interact at all-

“SANS!”

Oh, never mind. Sans lifted his head to see his brother marching towards them, looks like they’ll be doing the “puzzle bit” after all. He felt a small tug on his shorts which was accompanied by a small whimper; he looked down to see the kid holding onto his shorts for dear life, he’s not even sure the kid knew he was doing it.

Sans' smile waned at the sight, he really wanted to believe that this was all an act, a setup that Frisk had planned to throw him off guard. And he would’ve, if he didn’t have his judge ability. And right now it’s telling him that this kid had it rough, really really rough. Now this doesn’t stop him from being suspicious of the kid, he doesn’t even know why he’s here, but he knows that he ain't a bad kid. Who knows, maybe this run would be more bearable than the others.

Sans slowly reached down and placed a hand on the kid’s head. The child squeaked as he looked up at him shakily, Sans winked at him before he turned to his brother. “sup bro,” Sans greeted casually, Papyrus fixed him with an annoyed glare.

“DO NOT “SUP” ME BROTHER! WHY AREN’T YOU AT YOUR POST-?!”

Sans lifted a finger to his brother’s mouth and shushed him. He tried not to laugh at the taller skeleton’s offended gasp.

“hey try to keep it down, you don’t wanna to spook the kid do ya?” Papyrus’ face went from offended to confused. Sans pointed towards the kid who was looking at the taller skeleton warily. Papyrus gasped, he looked between him and Izuku so quickly he’s afraid that his head would fall off.

“SANS IS THAT A-!!!” Sans shushed him again.

“IS THAT A HUMAN?” Papyrus asked more quietly. He grinned when Sans nodded, “yup.”

Papyrus looked both excited and very confused, “I CAN’T BELIEVE IT I FINALLY FOUND A HUMAN!….BUT UH WHY ARE THEY SO SMALL…AREN’T HUMANS SUPPOSED TO BE BIG AND MENACING?”

Sans shrugged,“eh this ones just a babybones so i’m not surprised he’s not that big yet.” Papyrus immediately went quiet, a shocked look appeared on his face.

“....bro are you okay-”

“Sans please tell me your joking,” Sans was caught off guard by Papyrus’ quiet tone, it was odd for the skeleton to speak in anything but all caps, “Nah this isn’t a joke, the kid is an actual kid.”

Papyrus was making a face, Sans recognized that face. It as his “I’m about to have a very loud tangent about something that disturbs me” face. Yeah he gotta reign that in real quick.

“papyrus, before you go on a tangent I like to let you know that izuku here is very jumpy. so you're gonna have to tone it down with the yelling, we don’t want to scare him more than he already is,” Sans informed.

Papyrus nodded sharply as a muffled scream left his mouth before he turned around and marched into the tree line. Leaving the shorter skeleton and the child alone once again.

“Is he gonna be okay?” Izuku asked worriedly, any fear he had vanished after seeing Papyrus so upset.

“Yeah, he’s just need to let out some steam, he’ll be fine,” Sans gently patted the kid on the head, Izuku tensed a bit before he slowly melted into the touch. Welp looked like touch starvation needed to be added to the ever growing list of issues this kid had. Speaking of which, Papyrus might drag him away soon so he could fix his “puzzle”. Even though he’ll be gone for a short time, he doesn’t feel completely safe leaving the kid like that.

Sans sighed as he looked down at the little human, the kid’s eyes were closed as he had a content smile on his face. Sans hated to be the guy to ruin it, but he gotta do what he gotta do, “Hey kid, we’re gonna have to leave ya alone soon, are you going to be okay?”

Izuku's eyes blinked open before he looked at him with a determined look. It somehow wasn’t as unsettling as Frisk’s, “Yeah I’ll be okay, I’m used to being on my own!”

Wow, okay, yeah just say the most disturbing things like their normal why don’t you? Oh look that feeling’s back, lovely.

“Even so I'd like a way to contact ya, do you have a phone?” To his relief the kid nodded as he pulled out a familiar brick of black plastic. Looks like the kid had a run in with Toriel after all.

“Thanks kid, now hold on a minute…” Sans was quick to save his number on the kid’s phone before handing it back, “There, now if you’re in any trouble or you’re just confused about something, just call me.”

The kid was looking at him in complete disbelief and awe, his eyes slowly trailed back to the phone. He took the phone carefully, like he was waiting for Sans to snatch it back. How many times has someone done exactly that? How many times has someone done something so much worse? Stars, he barely knew this kid and he’s already fretting over him. Heh maybe this was Frisk's plan all along, get him attached to the kid only to take him away. Sounds like something the little demon would do. Welp he’s in too deep now, all he can do is make this day bearable for both of them. Stars knew they needed it.

It was just then that Papyrus came marching through the treeline straight towards them. He stopped a good two feet away from them before he crouched down. Even then Izuku still needed to crain his head up just to look Papyrus in the eyes, “HELLO LITTLE HUMAN, I AM THE GREAT PAPYRUS. IT IS MY JOB TO CAPTURE YOU AND SEND YOU OFF TO THE CAPITAL. BUT FIRST YOU’LL HAVE TO GET THROUGH MY MANY PUZZLES!…AFTER THEY GET RECALIBRATED.”

His bro waited until the kid said a soft “okay” before continuing, “ALRIGHT THEN, ME AND MY BROTHER WILL BE UP AHEAD CHECKING ON ALL THE PUZZLES, DON’T BE AFRAID TO TAKE YOUR TIME. AND IF YOU COME TO A PUZZLE AND WE’RE NOT THERE PLEASE STAY WHERE YOU ARE UNTIL WE GET THERE. SOME OF THE PUZZLES REQUIRE AN EXPLANATION BEFORE YOU SOLVE THEM.”

The kid looked a bit flustered by this for some reason as he replied, “I will.”

“GOOD,” Before he knew it, Sans was swiftly picked up by his younger brother. He chuckled a bit at the kid’s bewildered face, “THEN WE’LL BE OFF. WE’LL SEE YOU AT THE FIRST PUZZLE HUMAN, I THINK YOU’LL FIND IT RATHER SHOCKING. NYEH HE HE HE HE HE!”

The Tall skeleton hopped away carrying Sans with him. The shorter skeleton waved to the human child before he disappeared from view. He’ll check up on the kid later when Pap dropped him off. Hopefully the kid will be alright till then.
____________________________________________________________________________

Izuku watched the skeleton brothers hop away feeling shocked. That went way better than he thought it would. Both of them were so nice to him, even when it was their job to collect human souls they were very accommodating to him. He didn’t have to ask them either, they just did it! He knew Monsters were kind but he didn’t think they would be this kind!

Izuku looked down at the phone in his hands, that Sans just put his number in, just like Ms.Toriel did before him. He remembered how worried Ms.Toriel was when she found him in the Ruins, he also remembered how sad she was when he left ... .Izuku turned on his phone and clinked on her contact. He listened as the phone rang, but it only went to voicemail.

“...Hello, Ms.Toriel? I’m still alive. I met two nice skeletons, their brothers! I’m going to go solve their puzzles now. I'll call you when I’m done, Bye bye!”

Izuku hung up the phone and placed it in his key inventory. He looked up at the long snowy path that stretched before him. He should be going, he didn't want to make Papyrus and Sans wait, they already did so much for him already. With this thought in mind the young child made his way down the path, singing as he went.

“Harpy hare where have you buried all your children, tell so I say hmmm. Harpy hare where have you buried all your children, tell me I-EEP!”

His song was interrupted by an encounter, gentle blues and whites quickly blinked away to black. Izuku turned to the Monster who started the encounter, they weren’t one he had met before. They looked like a cross between a bird and a snowflake, they must be a Snowdrake. Snowdrake grinned smugly as they sent sharp crescents his way. Thinking fast, the young human summoned his bat out of his inventory. While he dodged the first wave of attacks he parried the next. Snowdrake looked surprised at this, “Dang man your really ‘snow’ joke!”

Izuku couldn’t help but giggle at the monster’s terrible snow pun. This got a genuine smile from them, “A laugh! I knew my dad was wrong!” The bird said triumphantly. Izuku smiled at the happy sight, he’s glad that he made the teen so happy. After he parried another set of attacks he clicked on the MERCY button, Snowdrake’s name glowed yellow. Izuku was more than happy to click it.

The world regained its color along with the happy Snowdrake, “Man wait until my dad hears this! See ya around human!”

“See you!” Izuku waved after the leaving teen before he made his way down the path. It wasn’t long until he caught sight of Sans, the skeleton waved at him, “heya there kid, i thought you’d come this way. i’m glad to see that i was right.”

Izuku blinked at the man in surprise, he was waiting for him? But wasn't he and Papyrus busy with their puzzles? Did he need something from him? He doesn’t have much to give at this point so he hopes not…But knowing his luck that’s probably it. That’s what most “nice” people wanted from Izuku-

“I was hoping to give you some advice for fighting my bro, and any other monsters for that matter.”

…Huh?

“By now I think you know about normal attacks that all monsters know how to use. But some monsters, especially my bro, can use something called blue attacks-”

Izuku immediately perked up, “Oh yeah blue magic! It can have many uses but it has two main effects. Those being gravity manipulation and movement trapping! Gravity manipulation is pretty self explanatory, but movement trapping is such an underrated ability! It allows the user to be able to create objects that can hurt/trap things when they're moving! This can have so many great uses like trapping bad guys, stopping things from clasping on people, and-” He slapped a hand to his mouth, dread filled his gut.

Stupid, Stupid Stupid STUPID! Everything was going so well why did he have to ruin it with his creepy mumbling!? Now Sans probably thinks he’s a creepy freak who knows too much. He can already see his disgusted glare without even looking at him, he’s such stupid stupid Deku!

“Wow, you really did your research huh kid?”

Izuku's breath hitched as he slowly looked up at the skeleton, his fear slowly turned to confusion when he noticed the lack of hatred on the man’s face. Sans was crouching in front of him with a curious look in his eyes sockets. Why wasn’t he mad? Everyone else always gets mad when he rambles, so why isn’t he mad? Before Izuku can voice his confusion Sans spoke again.

“Which is funny since most humans who come down here barely know how magic works, let alone the different types. Where did you learn all this stuff, kid?” Sans asked.

Izuku pulled on his fingers nervously, should he show it to him? Every time he tried to show it to someone they either didn’t care or tried to destroy it. He grimaced at the memories of the jeers and pain. No, no he’s not going to show it to him, not yet.

“My mama taught me, she said it’s “family knowledge” that has been passed down for generations,” Izuku answered simply. It wasn’t a lie, the only reason he knows so much is because of how his mama taught him. It’s just one of the many amazing things she’s done for Izuku.

“Really, huh your mom sounds like a smart lady.”

“Yeah she was…”Izuku cringed as he let that slip. Dang it why is he making all these mistakes today?

“................” Sans looked at him with an unreadable expression before nodding, “Well my statement still stands… Anyway you should probably keep heading down, knowing my bro he should be waiting at the first puzzle by now,” Izuku sighed in relief at the change of subject.

Sans stood up and stretched a bit, “I should be heading down there too, you ready to head off?”

Izuku nodded, “Good, see ya there kid,” Izuku watched Sans walk away, in the wrong direction…maybe it’s a shortcut? Izuku shrugged as he made his way further down the path. He was lucky this time, he really should be more careful. He didn’t want to lose anyone for any reason, he did that one too many times to make that mistake again.

He just had to be more careful.