"Ok.." You stare at the monster with determination. "I'm getting you out of here." He looked utterly surprised by your declaration. He shook his head. " T h a t I s I m p o s I b l e . "
"Hm." You thought on how you would do this. Then you recalled something Tori taught you. Soul synchronization. You summoned your [S/C] soul and the monsters.. Fragment? How could he live with only a fragment? You shook your head. It didn't matter. Toriel had said only to sync if it was an emergency or if you found your true mate. Syncing meant you could feel their emotions and vise versa. But maybe.. If you could sync with this piece and summon the rest of his soul he could leave? " W H A t a r e y o u d o I n g ? " He growled. Not that you could understand him anyway. You started to slow your soul beat to the pulse of the monsters. He sucked in a breath as your souls beat in synchronization. You saw it all as pieces flew in.
*****************

You couldn't move as you saw a skeleton type on a laptop. There was cracks in his just like the monster! Oh! Not far from him a tank stood tall, a young skeleton inside. "Sans.." Said skeletons tail twitched, drawing the older skeletons attention. He got up and walked through you, not past you, through you. "Can you hear me?" Sans, in response, opened his sockets. "Good, good. You've been out for a long time. Happy birthday." Sans pressed his skeletal hands against the glass. A panicked look in his sockets. He wanted out. "Calm down. I'll get you out. I just need to run one last check on your soul.." You frowned. Was this how monsters were born? Sans didn't seem to like that answer because as the monster turned he lashed his tail out. The sharp point jabbed through and the glass shattered it, making him and the blue water flow out onto the floor. The monster panicked and made sure Sans wasn't harmed in any way. "I'm Dr. W.D. Gaster. You are Comic Sans. Got that?" Sans tilted his head to the side.
********************

Sans was 5 now. He had started working as a lab assistant despite his extremely young age. You had to hand it to him though he was a very smart 5 year old. He was already doing algebra. Something you sucked at. Now Sans, Gaster, and you were looking at a tank-again- at another baby skeleton. This one was most definitely Papyrus. Turns out both of them were artificially made monsters. Their souls were for the most part artificial except the soul piece Gaster gave them so the soul would function normally. Pap's phalanx twitched a little and a smile spread. What? "hey papyrus!" Sans greeted the waking skeleton. Pap was much more patient as Gaster checked his soul. Than the orange liquid was drained and Sans held his baby brother. It was so adorable.
***************

It was a year later and Sans was working with Gaster and a yellow dragon monster named Dr. Alphys. She was very shy. Alphys left to get something when something happened. Sans grabbed at his chest with a hiss. Gaster and I both approached him. "Sans? Are you alright?" A low growl came from said monster. Oh no.. Sans lunged at Gaster who managed to block his feral sons attack. Sans' soul turned blue and he was pinned to the floor.
*****************

Sans was in chains. One on either wrist and one around the neck. "A soul imbalance.. How hadn't I caught this.."
"Sans broke out before you could." You tried to remind him. "To much magic to the soul.. If I don't fix this.. You'll be feral forever.." Sans snarled in reply. Gaster slammed his fist on the counter. "I. Will. Fix. This."
*****************

Determination you found out was bad for monsters, but he was injecting Sans with it anyway. You tried to stop him, but you just went through him. It was supposed to be his cure, but it didn't seem to work. He was constantly busy between curing Sans, raising Papyrus, and finishing the CORE. He was currently there now. Trying to fix some wires. One shocked him and he stumbled back, cursing in that language. That cost him. You wanted to catch him before he fell, but to no avail. From there everything was black. Over time he lost his form and occasionally he ended up in the grey room where he met Frisk.
******************

You snapped out of it as he reclaimed his soul and broke the connection. "I'll be taking that back, thank you." Gaster huffed. You.. Understood him. Not only that, but he had his original form back. "It worked!" You cheered. "Yes.. Points for that I suppose.. But don't touch my soul again. He grabbed your arm suddenly and you ended up in the lab. Alphys screeched in surprise. You waved and Gaster moved to the elevator. "Wait! He's not down there!"
"Where IS he?" Gaster demanded. "In Snowdin.. With Papyrus."
"What!?" Gaster raged. "Do you know how dangerous he is?!"
"You'll be dangerous if you don't calm down." You spoke evenly. You had him there. "I'll call him.. Ok?" You pull out your phone and called. "Call.. Him..?"
"HELLO! THIS IS THE DOMINANT PAPYRUS SPEAKING!"
"Hey. Um can you get Sans and come to the lab? I have a surprise for him."
"OF COURSE! WE ARE ON OUR WAY!" With that, he hung up. "Um.. I'm [Y/N] by the way."
"W.D. Gaster."
"A-Alphys.." Papyrus skidded in. "PAPYRUS!" He chirped, putting Sans down. "lil dizzy bro.."
"SORRY BROTHER!"
"so dizzy i think i'm seeing dad.."
"Sans..?" Gaster said shocked and hopeful. He oriented himself and looked up. Gaster was still there. "dad..?"
"Oh Asgore! Sans!" Gaster hugged his once feral son. "I HAVE A DAD?"
"yep."
"Come here, Papyrus." He quickly joined the hug. After awhile they broke away and Sans approached me. "i really owe you one. how..?" You shrugged. "Mmmmmmagic."
"huh.. um.. how about maybe.. you can tell me over dinner when you get to new home?" He asked as he rubbed his vertebrae. A golden hue on his cheekbones.

Do you accept his invitation?

Do you deny his invitation?