Undertale AU Sibling Oneshots

Undyne x Shy! Weak! Sister! Reader

Requested by the lovely lostabandonedsoul!~

"C'mon (Y/n)! You got this!"
You struggled on keeping hold of your weapon, the magic in your hands wavering as you focused on keeping it's shape. Your sister made it look so easy!!!
A few moments passed before your magic fizzled out, the breath you were holding leaving you in a defeated sigh as you hung your head.
"Almost!" Undyne sighed with you, patting your shoulder, "You did a lot better than last time!"
You gave an appreciative smile to her, blushing a bit at her praise. "Thanks, 'Dyne."
She gave you a toothy grin. "Don't mention it! Now c'mon, the bone heads are making dinner tonight!"
"Hey wait!"
Undyne broke off into a run, and you were left with no choice but to run after, the gentle mix of warm and cold of Waterfall changing to the icy cold of Snowdin.
Halfway there, your foot caught a snow pile when you tried to run over it, sending you face-first into the snow.
Your defeated, embarrassed groan was followed by Undyne's laugh as she pulled you up.
"You good, sis?"
Your response was to spit snow at her.

Dinner at the skelebros was nice, due to Sans helping Papyrus actually cook it correctly.
Frisk stopped by as well, so you got to catch up on all their adventures.
"(Y/n)!"
You perked up to the call of your name, looking over at Papyrus. "Yeah?"
"Undyne tells me you've been struggling with your magic! So I, the GREAT Papyrus, will offer to help you whenever you'd like!" He smiles brightly at you, the precious cinnamon roll practically radiating positivity.
It brought a warm smile to your face. "Oh thank you, Papyrus! I'm looking forward to it!"
Frisk then caught your attention, patting your arm with their tiny hands.

*Frisk tells you they want to help as well.

You let out a small laugh. "Thank you too, Frisk."

You were in Waterfall, following Frisk around as you both stepped through the field of echo flowers.
"So, you're planning on heading to Hotlands soon?"
Frisk nods, saying they want to free you all. It brings a small smile to your face.
"Thank you, Frisk. I'm glad to know you're willing to help." You ruffle their hair, stopping at the bridge near the end of Waterfall. They look back at you a bit confused.
"It gets warmer up ahead, not so good for a fish monster like me, y'know?"
They nod, giving you a hug before walking along the bridge.
As you watch, you tense up at the sight of an unstable part of the bridge ahead of them.
"Frisk wait!"
You run after them just as there foot steps on that piece and gives. You catch up to them, one arm looping around their waist as you both tumble off the bridge. On instinct, you fling your arm out, a spear shooting out of your hand as a chain attached to it loops around your wrist. The spear embeds itself into the rocks, and you fall a few more feet before coming to a jarring stop, dangling off the side of the bridge.
"Okay..." You mumble, trying to calm your nerves, "That's new... Are you okay, Frisk?"
Frisk nods, clinging onto you.
"(Y/n)!"
"Undyne!" You call out, seeing her come into view.
"What happened!?" She exclaims, moving to pull you both up by the spear chain.
"Bridge gave out." You reply, focusing on keeping your magic stable. Frisk reached up, touching your arms and helping you with a bit of their determination.
Eventually, Undyne pulled you both up, and then promptly picked you both up like to sacks of flour flung over her shoulders.
"No one is going over that bridge until it's fixed!"
You glance over at Frisk, who's giving you two thumbs up. Looking down at your hands, you still have your weapon in your hands, perfectly stable.