Undertale AU Sibling Oneshots

FracturedTale! Gaster x Reader

(*peeks out sheepishly* hi. I am return.)
(Ok so, technically in FracturedTale(my AU) canon, Gaster's madness is caused by morbid curiosity. In this, it starts that way but is ultimately pushed over the edge by your health)
(Yes this is angst because FT!Gaster is nothing but angst.)

"Up late again, Gast?"
"You're one to speak." Your brother spoke, not looking up from his notes. "You need rest due to your condition, and yet here you are, awake in the early hours."
Your condition, of course he'd bring it up.
From a young age, your bones were concerningly brittle, and stayed that way despite all the calcium medications you were prescribed.
"Come to bed, Gaster. Please, it worries me to see you go without sleep." You piped up again, leaning a bit further into the doorframe.
"No. My work is too important right now."
"You mean the work that will only hurt others?"
He lifted his head to look at you then, an offended expression on his face. "What?"
You frowned at him, fingers curling against the doorframe. "Your work is harmful. Fusing pieces of human souls with a monsters'?"
"My work makes us stronger, (Y/n). If you'd just let me do the same for you—"
"What would you do if this experiment of yours killed a monster?!" Your voice was rising now, "What if the piece of human soul is too much for our bodies to handle?! What if I crumble to dust the moment you try to fuse a soul into mine?!"
Gaster looked taken aback for a moment, and then his face settled into that neutral expression you've grown to hate. "I can assure you it won't happen. Now, go to bed, (Y/n)."
You grit your teeth at the sight of him turning his back to you.
"Why aren't you listening to me, brother?! Do you truly not care about the danger—?!"
"Enough!" He shouted, turning back to you as you grabbed his sleeve.
What was meant to be a simple grab to pull you off of him, ended up being too harsh of a squeeze on your wrist.
Crack!
The squeak of a gasp that left you nearly shattered Gaster's soul in that moment, the shock and panic written clear on his face. You were quick to pull back and out of his reach, your hand cradling your broken wrist bones.
"(Y/n), I—"
You stumbled back away from him when he reached out, stumbling back onto the floor and cracking something else in your body.
"No!" You spat, fear overtaking you now. "Don't touch me!"
It took you the longest to finally fully see it, but your brother was the truest definition of monster in this moment.
Gaster didn't move as you got to your feet, rushing out of the room. He shakily reached behind him for his chair, sinking himself into it before looking down at the hand that had harmed you.
He was shaking. He never shook, so why was he shaking?
Slowly he reached up to his face, finding tiny cracks around his eyes. Had those always been there...? Was he developing your condition now...?
He didn't know, but now he wished he had simply listened to you and gone to bed.
But the events that just occurred only told him what he knew all along. He needs to work quicker, and make monsters stronger with each human soul that falls down.