author's note: this became pretty successful on AO3 so i thought i'd publish it here too lol (p.s. i didn't really edit this so sorry for that i guess)
(note: in this world, soulmates are usually thought of as romantic, even if that isn't always the case. also, people change. soulmates can too.)
Toriel has never expected, in all the years she's lived, that her soulmate could change.
She and Asgore had been young and still living on the surface when they met, when she was still a princess and he was a brand new gardener at the palace. They had bumped into each other while she was taking a walk on the grounds and he was happily pruning the roses.
She remembers saying, "Oh! I am very sorry, I did not see you there. I was too busy looking at your flowers."
Years later, Asgore had told her that he was so happy and excited to meet his soulmate that he said, "It is a pleasure to meet you! Would you like a cup of tea?" Otherwise, he would never had spoken so insolently to the much esteemed princess.
He had, apparently, also forgotten that they were nowhere near his house and that he was still on gardening duty. Neither of them paid this any mind, and Toriel took him inside the palace to pour him tea instead.
When Toriel introduced him to her father, the man smiled very wide and clapped him on the shoulder, exclaiming, "I'm so glad to meet our future king!" (Because, at the time, queens were not a common practice and everyone much preferred to have a king at the reigns.)
When Asgore introduced her to his mother, she gasped and dropped the dish she was drying, because she never could have imagined that someone so humble as her son would marry a princess and - someday - become their king.
(It was never in question that two soulmates would eventually get married. It took a very long time for this opinion to change even the slightest bit.)
Later, they had a wedding. Asgore was crowned King and Toriel was Queen. This was supposed to be her happy ever after.
There was a war which Asgore didn't have the heart to fight but was forced to anyway. They were trapped underground, in complete darkness at first, but then they found crystals that would temporarily glow and they could see long enough to make lamps and torches. They built. They adapted. Soon enough the amount of monsters that remembered what the sun looked like could be counted on a five-fingered hand.
For a time she thought she could be happy. She was married, she had two beautiful children, and there was hope among everyone.
Then her children were killed. Asgore changed, promised revenge on the humans. He wasn't who she remembered him to be.
(She changed too, and that hurt Asgore just as much. She didn't know this.)
Toriel fled to the Ruins and locked the door, hoping to save any humans that might arrive. The black words were still twisting around her wrist and she hated to admit that they looked just as beautiful as they did the first time she could read them. She thought that they would be gone the next day, but they were still there. And they kept being there for the days, weeks, months, years afterwards.
Nine years and some months after she first left, she took off her bracelet. It was a while before she realized that the fur on her wrist was just as white as the rest. Her words were gone.
It took longer for Toriel to realize that this did not mean she didn't love him anymore, because the heartbreak was just as painful as before. The difference was that she doesn't have an excuse.
For the first time in many years, she broke down crying.
Unlike most people he knew, Sans wasn't born with words. They appeared much, much later in his life.
He had parents who raised him. He went to school at the appropriate age, he got a job at an appropriate age, and his family seemed pretty happy with this progress. He celebrated birthdays and holidays. He visited home at regular intervals. Sans was completely ordinary in every way.
Somewhere along the line, he managed to lose track of his family. But then he acquired a younger brother. He raised his younger brother. It was unclear to everybody if Sans and Papyrus were born in the underground or if they just decided to appear there later. Only Sans knows the answers and he doesn't really feel like telling anybody.
It was only when he was working as (or was it for?) the Royal Scientist that he noticed black words curling across his pelvis.
It reads, "Hello, I think we may..." He thought that was an odd place to stop, but he didn't question it. Maybe having a soulmate would be cool. He hoped they could be good friends.
Papyrus was a bit different. He's had words scratched across his humerus for a long time, and unlike Sans he would always talk about how he'll woo them, this person who says things like "Well gosh diddly darn apparently I got rattled by a skeleton" on their first meeting. Papyrus liked to kindly ignore how such a pun wound its way onto his humerus. Sans was a bit jealous of how such a pun wound its way on his humerus.
There were times after he first found those words on his pelvis that he'd freeze up whenever someone started a sentence with them. It'd be something like, "Hello, I think we may have mixed up our luggage," or, "Hello, I think we may have a problem here." He never noticed how common those particular configuration of words were until he found himself searching for them. But no matter what he replied with next, the person would smile, nod, and continue with the conversation without any indication that he said their words back.
After years of this it started to grate on him.
One day, Sans came home from work late, ready to read his brother a bed time story. Papyrus wasn't waiting in bed though, or anywhere around the house for that matter. He calmly, very calmly, took out his cell-phone and dialed his brother's number. He'll never admit to the relief he felt when Papyrus answered. Sans asked, hey bro, where are you? Papyrus said oh, I'm at Undyne's, you know the Royal Guard captain? Sans said well what are you doing there isn't it a bit late to be visiting friends? Papyrus said um actually we aren't exactly friends I was just asking if I could be part of the Guard. I'm going to wait out here until she answers. Sans said okay, good plan. Papyrus said don't wait up for me. Sans said sure thing bro.
Then he hung up.
There were always sentry positions open. Sans figured that he might as well support his brother in whatever way he can. Besides, it might give him an excuse to keep talking to that lady through the door.
Three children had come and gone. Toriel didn't know whether it was Asgore or his underlings that killed them, but it never made any difference to her. He was the one who ordered them dead in the first place, and in her mind that makes him guilty whether or not his hands are dirty. Most days, she likes to think about how glad she is to no longer have his mark. Some days, she admits that she can never regret being with him, not when she remembers feeling so complete.
It was around this time, between the third and the fourth child, that she first heard two distinct knocks on the door leading to New Home.
"Who is there?" she asks, because as far as she is aware no one ventures this close to the Ruins anymore, and also she is secretly hoping for a joke. Telling them to herself got old after a while.
"dishes," a masculine voice answered. Toriel smiled very wide. This was promising.
"dishes a very bad joke."
And it was, it really was, but she could hear the grin on their face even if she couldn't see it and she laughed and snorted and that was the first time in years that she feels very, very, happy.
"wait, i got another one for ya." They (or he, she supposed) knocked again.
"Who," oh dear, she needed to get her breath back, "who is there?"
"annie-one you want."
And that set her off again. He kept on telling her jokes and she kept laughing at them, even the ones she's heard before. Toriel didn't know how long this went on before she got the idea to tell him a joke instead. Before he could deliver another one, she knocked on the door.
"old lady who?"
Toriel had to fight to keep back premature giggles. "Oh! I did not know you could yodel!"
There was a brief silence before she heard him give a big, guffawing laugh, and she wholeheartedly joined in. The jokes became an equal exchange between the two, but eventually the person on the other side said that it was late and he had to leave.
"Wait! Will you please return tomorrow?"
"'course i will," he answered, "who would pass up an opportunity to tell jokes all day?"
He kept his word. It turned into something daily, something she could always look forward to. Eventually they transcended jokes, and began to talk about their homes, their family, their pies or their brothers. They knew everything about each other but names.
At one point, during this time period, she went to take a shower when she saw that some of the fur on the back of her left leg was black. Even without properly looking, she knew that she had new words, and she didn't know how long they had been there because she never made it a habit to check the backs of her legs. She had to get a mirror to see what it said.
"oh, hey, i recognize your voice too."
The words made no sense to her.
(She would never know this, but it was between the second and the third child that the mark first appeared. It correlated with the time she stopped feeling her heart break for Asgore.)
After officially becoming a sentry (at three different posts in varying places) it was basically his job to sit at the door and talk all day. Or, that was how he saw it.
It was also around this time that he started getting serious déjà vu.
Sans didn't think too much of it. So what if he felt like he was living the same few days or weeks over and over again? So what if he realized that he knew anything anybody was going to say? So what if he remembered seeing everybody turn to dust? It happened to everybody. It wasn't strange.
He started getting nightmares. A flower would talk to him, convince him to become friends with it. A flower would kill him. A flower would kill his brother. The voice behind the door would disappear and he'd never hear it again. Everyone would die. Everyone would be happy. It all blurred together until he doesn't remember what's real and what's not.
One day, Sans woke up to find that it was the beginning of the month when he thought it was the end of the month. That's okay. It was a common occurrence.
Except he managed to reach the end of the month, and he had to flip the page on the calendar. Sans nearly cried. Which was ridiculous, because there was nothing to cry about. A week passed.
The lady on the other side of the door had him make a promise, and Sans agreed. Soon after that, a human came through, small and striped and squinty-eyed. They kinda reminded him of a potato.
well, he thought to himself, there's nothing better than an introduction with a whoopee cushion.
Sans found himself making that promise over and over again.
Once again, Frisk had reached the end. They were certain that they made all the right choices this time - and even if they didn't, vines were blocking the elevator. They couldn't leave even if they wanted to.
All they felt was determination. This time, Frisk was going to spare Asgore. They didn't know how, but they would do it. Finally, they will get a happy ending.
Before the battle could begin, however, a burst of fire hit Asgore. Toriel had appeared.
This had never happened before.
She apologized to Frisk, and then Undyne appeared, and then there was Alphys and Papyrus and Sans and everybody was together. Yes, Frisk thought, this really must be the correct ending. Even though Asgore is in tears.
What they couldn't understand was the initial exchange between Toriel and Sans.
Toriel had recognized Sans's voice. Frisk could understand that. She said, "Hello! I think we may... know each other?"
Sans froze. That was very obvious to anyone paying attention.
"oh, hey, i recognize your voice too," he answered.
Toriel's paws went to her face in surprise. That made Sans smile wider and look excited. Seeing him look excited apparently made Toriel excited too. Seeing them both like this made Asgore cry more tears. Frisk felt bad for him and walked over to give him a hug.
They had absolutely no idea what happened.
The monsters were living on the surface. Papyrus was the one to explain soulmates to Frisk, after he asked them about theirs and they revealed that they had no clue what he was talking about.
It's why Sans and Toriel got together so quickly after they met, he said, they had each other's words.
But didn't they know each other before that? Frisk had asked. They used to tell jokes through the giant door that leads to the Ruins.
Oh yes, the rest of us were confused at first, Papyrus wisely told them, but apparently it's the words soulmates say when they meet face-to-face, not through a door or over the Internet. Which means all the dating websites I used to go on were useless.
What? Frisk asked. Dating websites?
I meant cats, Papyrus said. Then he ran away.
Frisk was very confused.
"Sans, can I tell you something?"
"you can always tell me anything."
"I know. I just wanted to say that while I was in the Ruins, after I discovered my new mark, I... well, I always kind of wished that it was you. I just did not believe that it could be, with what I was always taught. And, now, I'm glad that you really are my soulmate."
"heh. wow. thanks, tori. i gotta say, i, uh, felt pretty much the same. or i feel pretty much the same. i'm glad it's you."
"Thank you, Sans."
There was silence.
"Would you like to go stargazing with me and Frisk tonight? They tell me that the skies will be very clear."
"of course. i'll always go."
They never discussed what kind of relationship they had. They talked and joked and baked together and eventually they would live together but they never got married, and no one knows if they ever shared a room. No one knows if they've kissed, but everyone knows that they like to hold hands.
Sans and Toriel don't care. They are soulmates, and they make each other happy. That is a unique relationship in and of itself.
author's note: please leave a review or something. people on AO3 seem shy about that (in case you're curious in the week it's been out I've gotten 75 kudos and no comments :P)
edit- i forgot how infuriating FF was so instead of the pretty line breaks originally added in i had to actually type "linebreak" for something to stay there.