UnderKin is an AU that I created with my husband's assistance, I even added it to the Undertale AU wiki and created some art to post on my DeviantART: Gracie11facie

UnderKin is a world where most characters have extra friends or family members, and so I created this Fanfiction one-shot series to explain the history and intricacies of these characters in relation to the characters we know and love. Thanks for reading, and check out the art and wiki pages!

Story #1: The Littlest Skeleton


"Sans, you could have at least dressed up a little," Papyrus scolded, looking down at the older skeleton, who hadn't changed at all from his usual attire.

"Sorry, bro, guess I just didn't want to do a skele-TON of work," Sans replied. Papyrus gave a disapproving scoff and crossed his arms over his freshly pressed dress shirt. "C'mon, bro, don't let this stuff get under your SKIN."

"It's just not the time for puns, Sans. I'm sorry, but I'm just...I'm not in the mood."

Sans didn't say anything, and they walked in silence for a few minutes. It wasn't like he didn't understand, because it was a hard day for him, too. But he was just built different, and always covered up his pain with jokes and silliness. Papyrus wore his emotions like accessories, and Sans honestly admired that. He turned his gaze up to his brother, watching the waves of emotion hit the taller skeleton: Angry, sad, frustrated.

He let his eyes fall back to the ground in front of him. Gaster would have told him that emotions can be weaknesses which our enemies exploit, and it's good that Sans can control them. But Gaster wasn't here anymore.

"Brother?" Papyrus' voice pulled Sans from his reverie.

"Yeah, bro?" He tried to sound light-hearted, but it was probably not as sincere as he thought it was.

"What do you think he wrote in there?"

"I dunno, bro, guess we just gotta wait and find out."


Papyrus squirmed in his seat. The stern lady that had answered the door to Gaster's lab was now sitting across from the brothers, peering over the stack of papers in her hands: Gaster's last will and testament. Sans was leaning back in his chair nonchalantly, and it took all his power for Papyrus not to get into it right now. Really, his brother may be older, but he always felt more responsible.

"Alright, most of these pages are claiming rights to some of his research to Alphys, the succeeding royal scientist, however," The monster lady tapped her claw on the page. "All the physical items in his lab are to be requisitioned to his three children. Now, the king would like-"

"Ah, um, excuse me, my dear, but I think you read that wrong," Papyrus interjected politely. Sans lowered his seat back on all four legs.

"Yeah, lady, there's just the two of us," he said, a look of concern showing on his face. Finally, a genuine feeling! Papyrus couldn't be prouder to have caught that look, and so he overlooked the smaller brother's rudeness.

"Hmm…" She flipped through a few pages. "Well, it is just the one mention, so I suppose it could be a typo. In any case, you have until the end of the day to remove the items you wish to keep, as the new royal scientist needs to move in today. Any questions?"

Papyrus had a lot of questions, but figured he shouldn't ask them. Instead he shook his skull no, and he heard Sans grunt in agreeance. With that, they signed some papers, and the lady left.

Papyrus waited for a pun, but didn't hear one. Curiously, he looked over at his brother, who was busy staring around the room. Sans used to work with Gaster in the lab, whereas Papyrus never had the pleasure. He figured he must be experiencing some melancholy at the thought of wrecking the place.

"Come, brother, we'll take all the good memories home with us! The Great Papyrus must have all the amazing memories of our father there is to have!" He struck a pose, then snuck a peek. Sans chuckled, swinging off the chair onto his feet.

"You got it, ya BONE-head." He teased, winking at him. Papyrus felt a weight lift off his shoulders as the older skeleton walked away, calling out to him that he would start in the lab if Papyrus would do the office.

"I'm still in doubt that you'll do anything at all!" Papyrus shouted at him as he left the room. He smiled, knowing that he was very good at reading his brother, despite what Sans might think.

He clapped his hands together and looked around the office. He had to start somewhere!


Sans flopped into the office chair, allowing it to swing freely, and let out a huge sigh. He'd been working harder than he had ever worked for the last few hours, and all he had to show for it were a few choice science things and a small pile of papers and drawings Gaster had kept around the lab like family portraits. He eyed the huge pile of things that Papyrus had stacked and wondered if it was all necessary, as he didn't really want to carry it all back to Snowdin.

"Oh, Sans, this is the first break I've seen you take! You are working very hard!" His brother beamed down at him from next to the chair. He felt a bit better, knowing he was making Papyrus proud.

"Not nearly as hard as my cool younger brother, heh…" He grinned up at the taller skeleton. "So, have you found anything worth keeping?"

"Oh, lots of things! Gaster kept many useful things, such as old receipts so we could know the prices of things if we ever need to return them, and also this glorious device that sticks paper together!"

"That's a stapler, Pap."

"Nyeh! A stapler! It will certainly be useful around the house, then!" He shuffled around for a bit, and then handed Sans a stack of papers. "I also found these baby pictures of us when we were just little skellies in our development tubes!"

Sans shuffled through the pictures with a grin. They were so little inside their giant test tubes, and in each one Gaster is seen hard at work on their souls. He can even see the faint blue or orange light from inside their ribcages, and himself in the background of one of Papyrus', grinning up at his future brother.

He paused as his finger bones had fallen on a picture he hadn't seen before. A haggard Gaster staring into a tube where something is obscured, glowing...purple. He scratched his skull. He didn't remember any other experiments after Pap.

"Oh, and Sans, there's a bust on the desk that I want to take but for some reason I can't pick it up. Could you use your gravity magic to get that?"

Sans glanced over to the lopsided, smiling Gaster head that he'd made out of clay when he was little. The old man had coveted the thing, almost as if it were some truly prized treasure instead of a child's art project. He smirked.

"Yeah, I'll get that for ya, Pap." He flicked his wrist, lifting the bust off the desk easily. As he deposited it in Papyrus' pile, he heard a series of small clicks. "Uhh...Pap?"

Sans gripped his chair with one hand and Papyrus' scarf with the other as the floor fell out from underneath them, sending them on a chair ride down a long, dark hallway. Papyrus started screaming as he wrapped his bones all the way around Sans, causing them to fly off once they hit the bottom and crash into the wall.

Sans sat up, disoriented. He thanked his brother for the "sweet catch" and looked up from their spot on the floor.

As he looked around, he recognized the area vaguely from memory as well as pictures. This was the soul synthesis room, where Gaster had hand-spun his and Papyrus' souls.

"Wh-where are we?" Papyrus asked, unable to hide the chatter of his teeth and bones.

"This is where Gast-uh, where Dad made us." Sans stood, squinting at the far end where he could see a faint glow. "Hey, hit the lights, would you Bro?"

"Oh, uh, of course."

After a few seconds of fumbling, light filled the chamber, illuminating the workbench and test tubes that filled the small basement. There was one test tube that had fallen on it's side, possibly with the impact of the brothers hitting the wall but he couldn't quite tell. Faint light poured out of it from underneath a fabric cover.

"What is that," Papyrus gasped. "That light looks like a-"

Sans swiftly ripped off the cover, but was for once struck speechless by what he found.

A small skeleton floated in the chamber, purple soul glowing dimly in their chest.


Sans tore through the papers on the workbench, looking for anything related to the small skeleton. Papyrus seemed to catch on to his panic, because he was worriedly hovering around the chamber, wringing his hands and chattering nervously.

"Don't worry, lil' Bones, Sans and I will figure something out! Sans, should I, eh, take them out? Or something?"

"No!" Sans swivelled around and threw his hand out to motion 'stop'. "No, no, her soul is too weak, if you take her out now she'll die." He turned back to the papers, his hands finding purchase on some very messy notes. "She's been left unattended for weeks now, her development is already in danger. Maybe there's something in Gaster's notes here that can help us…"

Papyrus watched Sans mutter to himself seriously and wondered what had happened to his usual levelheaded brother. Granted, he wasn't nearly as scientifically gifted as the elder skeleton, but Sans never acted like he was smart so to see him so focused was strange. He returned his focus to the tiny form inside the chamber.

"So it's a girl?" He asked.

"What?" Sans replied gruffly, shuffling papers.

"You said 'she'. So she's a girl? How do you know?" The sound of shuffling papers stopped momentarily.

"I, uh...I dunno, actually. I can just...feel it."

He tried to see the face, but it was in fetal position so it was difficult. What he could make out was a sizable crack in her skull. He felt worry in his soul. What if the little one didn't make it? What if the damage was irreversible?

"Don't worry, little sister, the greatest big brother Papyrus is here for you! Everything will be a-okay!" He thought that maybe these words were more for himself, but it did make him feel better. He felt Sans tense up behind him.

"Papyrus, uh...just, don't get too attached, okay?"

"But why not? She's our sister, and she's relying on our help. With you and me together, we can do anything, so little sis has nothing to worry about, right?" He beamed up at the older skeleton, who hadn't turned his way. He saw his shoulders droop.

"Yeah, Paps, don't be such a numbSKULL."


Sans stared into the container, clutching Gaster's notes in his hands. Her name was Bree Serif, Gaster's perfected recipe for a monster. The crack in her skull had marred what was supposed to be a flawless addition to their family. He cast his eyes down at the notes, ashamed to look at her. Alphys had been nice enough to help out when she showed up, so the small skeleton was in stable condition, but there's no telling what kind of issues she might have because of the damage caused when they toppled her container.

Paper crinkled as he tensed up, frustrated with himself. He knew he was acting out of character, but this was family they were talking about. She was so small…

He saw stirring out of the corner of his eye and looked up. Bree Serif had stretched out of fetal position, turning her skull upwards so her soul could shine vibrantly from her chest. A good sign, it meant she was progressing.

"Hey there, Lil' Bones. I'm your big brother, Sans," He whispered through the glass, knowing she probably couldn't hear him. "Your other big bro is way cooler than me, but he's gone right now to prepare the house for if...for when we get to take you home."

He set the papers aside, freeing his hands so he could place them on the glass.

"You're gonna love him, kid. He makes spaghetti every night for dinner, and he'll play with you

whenever you want. He's really excited to be the big bro for once, you know? He's so good that he's probably the only reason I still exist. Well, I guess I got two reasons now, huh?"

There was a clatter behind him and he whirled around, Gaster eye glowing, only to see Alphys dusting herself off.

"S-Sorry to sneak up on you, S-Sans, I just thought you'd like to hear the good news."

"Good news?"

"Yes," the dinosaur said, smiling up at him as she walked over to the tube. "Her d-development is going smoothly, and she should be ready to go h-home in a day or two. Oh, and I don't think the crack in her skull is much to worry about, i-it should heal by the time she is full grown. I ran all the numbers and that is the m-most likely outcome."

Sans stared at his friend, dumbfounded. He'd been so worried over nothing. His little sister was coming home, and she'd be better than okay. He glanced back over to the small skeleton pensively.

Suddenly he groaned, realisation crashing down on him.

"I, uh...I gotta get a job."


Sans put his hood up, waving goodbye to Grillby as he left his evening shift. Grillby was always a great pal, and had offered Sans a job without a second thought. Lucky for him, he could pretty much just stand there and wipe the same glass all night and nobody would be the wiser.

He stepped out into the snow, his breath making clouds in front of him as he walked, slowly at first, and then faster.

Papyrus had brought the little one home that night. He was finally going to meet her for real.

His slippers hardly touched the cold ground as he flew towards his house, excitement racing through his bones. He remembered this feeling, the same feeling when he met Papyrus. The feeling of having family to live for.

He threw the door open, depositing his slippers flawlessly by the entrance. The living room was a huge mess, but he didn't even see it. What he did see was the most precious sight he had ever seen in his life.

Papyrus sat in the middle of the mess, cradling the baby skeleton in his arms. A familiar lullaby rang out from his mouth, the same as Sans once sung for him every night. The tiny finger bones wrapped around Papyrus' huge gloved finger, the big violet eye looking up at him adoringly. He didn't even notice that her right eye was unable to open due to the crack, she was the most perfect baby skeleton he'd ever seen. This was his family.

The singing stopped, and Papyrus' concerned voice called out to him. "S-Sans? Are you okay? Why are you crying?"

He hadn't even noticed the tears flowing down his cheekbones. He crossed the room swiftly and brought his siblings into a gentle hug.

"Sorry, bro, guess I'm just a big BABY, huh?"


A/N: I hope you enjoyed the introduction to Serif's background, she was a pivotal part in the creation of UnderKin and so it is only fitting her story was told first.