Undertale: the human equation

- hey there. Welcome to my little undertale fanfic. I have no idea how long this story will go, but updates should happen bi-weekly. Generally Mondays or Tuesdays will suffice, but we'll see. This will not involve any of the other undertale fanfics, such as underfell or others. I will put a trigger alert here, however, because I am relentless with my writing and will probably dabble into some darker stuff later on. This first chapter should be light reading for all of you, and I hope you enjoy. I will switch back and forth between San's perspective and Frisk's. It just seems more fun that way. I'll probably make some art, too. It'll all be hand done, as I'm not the best with a tablet.

Chapter 1: the decision

Golden sunlight fell upon green, grassy lands. Silhouetted against the sun were mountains in the distance. The sun rose to greet seven creatures standing alongside a giant hole in the ground. A great battle had ended, and the creatures were rejoicing in their newfound freedom. As happy as the scene was, one such creature stood in uncertainty.

He was a short skeleton with a lot of magic at his fingertips and even more worry in his chest. The king and queen of the underground, two goat figures stood beside a human with smiles on their faces. Alphys the pudgy lizard woman stood near her new lover Undyne, who held her scaly fist against the sun. Papyrus laughed and patted the human on the back, stating loudly that Frisk couldn't have done anything without his help.

Sans, of course, knew better. He had watched the different timelines form when he saw the time anomalies in his lab back at the house. He had even shown his brother, in an act of desperation, when he feared that the human might try to kill them.

But she hadn't. Frisk hadn't killed a single creature, even the enraged dummy. She had spared all of them, thus saving them without fighting at all. Frisk opened her mouth wide and laughed at Papyrus, before turning her head to look at Sans. She was blind, so she couldn't actually see, but somehow Sans felt that she could sense him, and everyone.

Sunlight illuminated her small face as she grinned.

"I, the great Papyrus, will be the best mascot," Papyrus said triumphantly. "I'll go make a good first impression!" He quickly ran away into the nearby forest.

As usual, the power of the human sent chills through San's bones. He kept his gaze locked on her, before winking and grinning. "Someone's gotta keep him from getting into trouble." He turned on his heel and headed towards the ruins. Sans did this for two reasons: the others, aside from Alphys, would find it funny. He also wanted the human to know that he, too, had certain powers beyond what the others could do.

In truth, Sans needed to exit the scene. He wasn't upset, if anything he was incredibly relieved, but he knew as well as Papyrus did that if the human really wanted to, they could go back and erase everything. San's magic burned blue and he teleported back to his house, in Snowden.

A year had passed. During that time, Frisk and Toriel made frequent visits to the surface. Toriel never admitted it, but she went to protect Frisk from other humans. They had began speaking with the chief of the nearest village, who were responsible for the wars anway. Papyrus had gone with them once or twice, to make jokes and supposedly to cook. Sans teleported once just to check, only to see Frisk and the humans smiling and happy with Toriel and Paps.

At the time, no one had noticed Sans, and he was about to leave, unnoticed. But, right as his magic began to burn at his fingertips, Frisk turned her little head and caught his eye.

She smiled and waved, and even beckoned Sans forward. Sans felt his bones tense at the sight of the girl. He couldn't even shake his head, he merely teleported home and collapsed on the floor of his dirty room, clutching his clothes.

I don't have a heart, he thought, but would it be beating fast?

It had been a year, full of surface visits, monsters moving out from the underground and humans moving in, and random, new encounters. The human came to see him, sometimes, and Sans did his best to make things fun for the child. He had noticed, however, a little yellow flower sometimes and grew worried. Ultimately, needing space, he isolated himself to his room.

But, of course, he couldn't really be alone.

The dirty floor of his room suddenly cracked and groaned and a flower burst through. It looked left and then right.

"Yuck, when was the last time you cleaned in here?" Flowey asked.

Sans shrugged. "I guess I just don't have the heart to clean it."

Flowey snickered and moved forward. "Listen, here, bone head, I've been watching you."

"For someone who watches the timelines as much as I do, how can I say I'm not surprised?" Sans replied dryly.

Flowey's normal grin fell to a hard frown. "I need to ask you something."

Sans sighed and closed his eyes. "Sure, but can I take a nap first?"

"No, dammit!" Flowey moved his roots and used one to snap at Sans's legs.

"Hey now, no need for that," Sans said, rubbing his leg. He winked. "And here I was, thinking you had grown a little."

"Ha, ha, wordplay," Flowey said, wiggling his stem. "Hey. Do you have a crush on Frisk?"

Sans froze and felt his bones tremble. Flowey was the only person who could warp space and time other then Sans himself. If Sans answered in the wrong way, Flowey might try to destroy the entire timeline.

"Your stunned look is telling me yes," Flowey speculated thoughtfully.

Sans's heart fell and he shook his head. "You've got it all wrong."

"I am? Wow, and her I was about to warn you that she's marching over here to confess," Flowey said with a wicked grin.

Sans froze again and his chest tightened. She's gonna confess? That sounded similar to something Alphys would watch in her lab.

"You do like her!" Flowey cried.

"Hold on," Sans started.

"She's just a kid!" Flowey spat. "You're disgusting.'

Sans was quick with his words. "Well," he said with a laugh, "I'm not really that old, I mean technically I didn't appear until about two years ago."

"That's a flimsy excuse, besides," Flowey was grinning awfully again. "You're a skeleton. You'd never be able to be with her."

Flowey was saying something that San's hadn't quite come to terms with. His heart fell and he looked at his feet. From outside of his room, Sans could hear Papyrus at the door.

"Human! You've come for a visit! And I, your best friend, the great Papyrus, will entertain you in the best of ways!" Papyrus stomped through the living room.

Ah shit, she really is here, Sans thought in a panic.

"Is Sans here?" he heard Frisk's small voice.

"If he isn't being a lazy bones!" Papyrus yelled.

Sans put his hand on his head and tried to think.

"Good luck, lazy bones," Flowey said with a malicious laugh and left, leaving a little hole in the center of Sans's floor.

"Paps, could you make some spaghetti? I'll go get Sans," Frisk said.

"Yes! And it will be the most fantastic spaghetti you will have ever had! Even better then the frozen spaghetti I left on that table!" Papyrus cried, cackling and walking into the kitchen.

Sans heard Frisk walk up to his door. She knocked softly and waited.

Sans momentarily contemplated just ignoring her, before he noticed a brown root rise out of the floor and turn the handle.

Damn flower, Sans thought.

Frisk stood in her too-large shirt with a cheesy smile on her face. Most other monsters either thought she was a boy or didn't bother recognizing her gender, but Sans knew.

"Hey bone head," she said with a toothy grin. She stepped forward confidently.

"Hey, kid," Sans said, doing his best to appear normal.

Frisk sat beside him on his bed and stared at his swirling vortex of paper and garbage in the corner. "You didn't come to my birthday party."

Shit. "Ah, kid," Sans said. "I'm sorry."

"I can't forgive you," Frisk said stubbornly and crossed her arms.

Oh, no. Sans felt miserable and looked around his room, and thought of all the places he could go in the underground to get her a gift.

"At least, not unless you can answer me one question," Frisk went on.

"Alright," Sans replied cooly.

"Who has two thumbs, and likes you a whole bunch," Frisk asked. Her cheeks ran red.

Sans was taken off guard and stared at the hole in his floor where Flowey had been. Before his own assumptions could get the best of himself, he stuck out both thumbs and pointed at himself. "This guy," he said.

Frisk giggled and shook her head. "It's me, you dope," she said and playfully punched Sans on the shoulder.

Sans sighed and continued to stare at the floor. "Kid," he started.

Frisk hopped off of the bed and walked towards the door. "That's all I wanted to say," she told him. "No need to get mushy, I know I'm quite the catch." Frisk grinned at Sans before she left.

Sans lifted his finger and his door shut. "Kid, we need to talk."

Frisk blushed and nodded, but didn't move from the doorway.

"I'm a whole heck of a lot older than you," Sans said.

"I don't care," Frisk started but Sans held up his hand.

"Well, I do," Sans said sternly. "It doesn't matter if we like each other. We can't do anything."

Frisk pouted and looked away.

"Second...I'm not human, Frisk." Sans sighed and felt his body sag. "Even if...I'm not so old, my memories and my knowledge is. All the things I learned working with Alphys and Gaster...all the timelines I know of. I'm not human, and it wouldn't be fair. I could use magic to simulate what humans do, but...it wouldn't be the same. Especially with you being so young."

"I'm mature for my age," Frisk argued. "Besides, I'm officially a teenager. I'm thirteen now!"

Sans stood and patted Frisk's shoulder. "Kid. If...you really want to be with me, then you're gonna have to wait a least five years." Sans used his magic to open the door. "And that's a long time."

"I'll do it."

Sans froze and stared at Frisk.

"Kid," he started. "Don't."

"Are you saying you don't like me, then?" Frisk asked. Her little fists clenched and she stood her ground.

"I'm saying that for you, five years is a long time," Sans insisted. "You'll have plenty of boyfriends and I don't want to get in the way of that."

"You won't," Frisk said determinedly. "This is my choice."

Sans stared at her, tempted to hug her or kiss her, but he steadied himself. "If you change your mind, it's okay." Sans gestered for Frisk to leave.

Frisk stepped through the doorway and paused. Both could hear Papyrus singing a song downstairs.

"Sans?" Frisk said, and turned. She opened her eyes, even though she couldn't see.

"What's up, kid?" Sans asked.

"I love you."

Sans froze again, only this time, the urge to kiss her almost overtook him. Still, something in him cringed at the wrongness, and his own morals screamed the reality that she was just a child. A small voice in his mind said: so are you, but he ignored it. Sans knew better. And as he stared at the child, pouring her feelings out to him, he knew that he had to do something.

Sans reached forward and patted Frisk's head. "Kid...I love you too. Now stop being silly and quit ignoring Paps."

"Human! The food has been prepared!" Papyrus announced from the kitchen.

Undyne had once told Frisk that she had to be assertive. This was not uncommon or even unknown to the young girl, because in order to save Aserial, she had to continue standing up for herself. So, when she realized that she loved Sans, she did the only thing she knew she should do. She told him. And she received no answer from the short skeleton.

Frisk wandered the ruins the next day, talking with the frogits and laying with Nabstablook. She had gotten him a monocle which he adored and wore with his hat when he was in good spirits. After the ghost creature faded away, Frisk found herself walking alone.

Though some monsters now lived on the surface, and some humans now lived underground, the ruins remained empty for most except for Toriel. She and Asgore had been speaking for several months, and sometimes even going out to Grillby's, but she remained in her own space instead of living with him. Frisk never questioned this, because it seemed like a sensitive subject. Even so, Frisk liked to pretend that someday the king and queen would be together again.

Toriel sat in her chair next to her fireplace, reading a book. Her furry nose was always cute to Frisk, but as Frisk sat down, she couldn't relish in anything.

Did Sans like her? Or not?

He had said he loved her, but was that more of a family statement, or did his feelings burn as strongly as Frisk's?

Toriel stood and gracefully walked to Frisk's side. She put a fuzzy paw on the child's shoulders.

"You look troubled," she said gently.

Frisk stared at the kind Toriel before letting out a long sigh.

Toriel nodded, as though she expected Frisk's actions. "Would you like to talk about it?" she asked, settling in her chair once more.

Frisk tapped her fingers on the table. "I told Sans that I love him."

Toriel coughed and dropped her book. "Oh," she said, with raised eyebrows. "Really." Frisk couldn't help but detect a hint of anger.

"He didn't do anything," Frisk said dryly.

"I would still very much so like a word with him," Toriel said flatly, putting her chin on her fist.

"He said that he was too old and that he wasn't human so we couldn't do anything," Frisk said. Her nose felt hot and she turned away to hide her stinging eyes. Toriel wasn't looking, but Frisk was already uncomfortable talking about her feelings to what she considered as her mother.

"Even so," Toriel said. "He is...kind, but he can be very adult regardless of his actual age."

"His actual age?" Frisk asked.

"He and Papyrus just appeared one day," Toriel said. She offered a sly smile. "He came to my door and began telling me jokes after that."

Frisk sighed and shook her head.

"Oh my sweet child," Toriel said. "Not all romances happen the way we want them to." The wisdom in her eyes made Frisk a little sad.

"I know we could be happy together," Frisk said stubbornly. "Why is that bad?"

Toriel sighed and ran a hand over the top of her head. "Sans...is doing what is best for you."

Frustration blossomed in Frisk's chest. "I know what's best for me."

Before Toriel could answer, her phone wrang. She hesitated before answering. "Hello?" she cooed as she stood. Toriel walked to Frisk and patted her shoulder.

Frisk watched curiously, hearing the sharpness of Alphys's voice. As Alphys spoke, Toriel's brows rose.

"What? But…"

Alphys went on, and Frisk could practically hear the anxiousness in Alphys's voice.

"Alright. Well, call...yes, sorry, text Asgore immediately." Toriel looked at her phone with furrowed brows.

"Tori?" Frisk asked. "What is it?"

Toriel looked stricken at first, before pocketing her phone. "Nothing, my child. I've made some butterscotch pie. I'll bring a slice up to your room if you'll wait there for me."

Frisk nodded, and trudged through the house while Toriel began to text quickly. Frisk pulled out her own phone, and texted Alphys.

What's going on? Frisk was sure she'd wait for a while before she got an answer. To her surprise, Alphys answered immediately.

Sans is trying to become a human.