A/N: My headcannon theory about Gaster. Spoilers kinda, implied child abuse. This was supposed to be happy but I started writing and got carried away and I'm so sorry my poor babies I love you please forgive me I'm a horrible person.
Also why is Gaster not an option for character selection on here?
Sans hadn't necessarily been an accident, but the child's existence nevertheless surprised W.D. Gaster. He'd been experimenting with monster Souls, trying to create one to impress King Asgore, even using himself as a subject, all hoping to reach his dream job as Royal Scientist. He hadn't expected a child to come from it. Even more surprising was how quickly he became smitten with little Sans.
Not long after, Gaster was hired to be the assistant to the current Royal Scientist.
Eventually he decided Sans needed a playmate, a friend.
"Sans," Gaster knelt, lowering the bundle in his arms to the boy's eye level. "meet your little brother, Papyrus."
Sans blinked at the little skeleton a few times before a huge grin spread across his face.
Gaster told the children a bit of what he worked on; Sans was intrigued, but Papyrus was uninterested.
Life was good for the little family.
Gaster seemed very happy one day; he was humming as he made his extra special spaghetti.
When they all sat to eat, he cleared his throat. "I have some news boys," he smiled, "today I was promoted to Royal Scientist."
"COOOL!" Papyrus kicked his feet excitedly. He actually had no idea what that meant, but it sounded great.
Gaster's smile faltered. "However, I'll be working longer hours,"
Sans frowned. "Will you still read us bedtime stories?"
"Of course, boys."
"Will you let me help with work?"
Gaster paused in thought. "Perhaps,"
Gaster instructed Sans in some of the science he used, and let him help with simple things in the lab. The Core, time travel, teleportation; the boy was fascinated by all his father's plans.
Unfortunately for everyone, Gaster had not realized how taxing his new job would be, with all he wanted to accomplish. He still let Sans assist, but sent the child home early as he started staying at the lab later and later, becoming obsessed with his work.
He read the boys less and less frequently, often falling asleep in the middle of the stories.
One night, as Papyrus lamented the lack of bedtime stories, Sans decided enough was enough.
He made his way to his father's lab, and standing in front of the door, hearing beeps and low muttering from inside, Sans began to lose his resolve. But he set his jaw and slowly pushed the door open.
"Sans?" Gaster sounded shocked, then annoyed. "What are you doing here?"
"You were s'posed to be home hours ago," the boy said quietly, looking up to meet his father's eyes. He was angry. "Pap wants a bedtime story."
"You can read, can't you?" Sans flinched at the loudness. "I have a lot of work to do."
And that was the end of it. Gaster hunched back over the papers on his desk, and Sans left, replacing the door to its previous position.
Cracks began appearing on Gaster's skull, a physical reflection of the cracks in his sanity.
Gaster came into the boys' room one night without knocking.
"Come with me, Sans," his voice was low, almost hollow. "You're going to help with some experiments."
Sans set the book he'd been reading onto the bed next to him and stood. He'd given up a while ago; the only thing he cared about now was keeping Papyrus safe. Sans didn't tell Papyrus anything about the lab anymore, and although he was a bit oblivious to other things, Papyrus understood that 'help with' meant 'be used in'.
Gaster took his son's hand.
"W-wait," Papyrus hopped off the bed and stood as tall as he could, which admittedly wasn't very intimidating even though he was almost as tall as his brother. His voice shook. "Take m-me instead,"
Sans started to protest, but Gaster cut him off.
"Papyrus, you are not strong enough."
Sans reassured his brother that he would be the Great Papyus one day, and could do anything he wanted; like be Captain of the Royal Guard. Papyrus loved this idea, because then he'd be able to protect his brother.
He didn't know about the experimental strengthening formula his father had been occasionally slipping him.
One day, the boys felt the ground rumble beneath their feet.
A few minutes later the door slowly opened. Sans pushed Papyrus behind him, even though his little brother would be able to see over his head.
Gaster stood in the doorway, hunched over. He was melting and flickering.
"Experiment went w-wrong. Core-" the man stammered, "don think you- you'll 'member me," he huffed a laugh; "prob-ly a good thing, I been horrible to y-you." He looked up at his terrified sons. Terrified of him. "Stay- stay strong. Keep s-smiling."
And then W.D. Gaster was gone, and the boys didn't know why they were standing like that, staring outside their door.
But Sans remembered a little of it all. Mostly vague flashes in his nightmares.
He held his interest in science for a while, even after he and Papyrus moved to Snowdin. But eventually he gave up, he'd never even been sure of what he was trying to accomplish.
"Sans?" a small voice said. "You ok?"
"I'm good kiddo." He coughed lightly. "Anyway… I have a request for you. I kind of have a secret codeword…"
