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Bad Brother AU

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Undyne glanced up from her piano when she heard soft, hesitant knocking at her door. Who the hell was rapping at her door at- she checked the clock on the wall- nearly 11 at night?

She rolled her eyes fondly as a name came to mind. Papyrus liked to turn up at her door at odd hours to beg her for training, and while his enthusiasm was awesome, he really needed to stop showing up at strange hours. Although it was strange that he was knocking so gently...

Getting up, she fixed her eyepatch before throwing the door open, ready to exclaim a faux-annoyed greeting before stopping at the sight of Sans, who was nervously shuffling his feet.

What was Sans doing here? Although they were friends and occasionally hung out together (and by that she meant he appeared out of literal nothingness to talk sciency crap with Alphys), they were nowhere close to going over to each other's houses yet. And...

Sans was kind of...weird, to say the least. The small skeleton had none of his brother's cheerful optimism and exuberant personality, and definitely none of his height. She just didn't understand it.

Sweeping a critical eye over Sans' slim figure, she noted the alarming bruises marring his skull, along with how his hands trembled slightly. Now what the hell had happened there? She'd definitely need to talk to Papyrus about that.

"uh...undyne?" The skeleton's soft voice jarred her out of her thoughts, but she carefully controlled her reactions as she grinned down at him, flashing her shark teeth.

"What's up, punk? Where's your bro?" Undyne didn't miss the way he flinched at his name, eyelights darting to the side to avoid her gaze, but wrote it off as anxiety from talking to the Captain of the Royal Guard. She was pretty freaking COOL, if she did say so herself.

"y-yeah...that's kinda what i came here to talk to you about..." She huffed silently, hands on her hips. Why was he so freaking formal all the time? In an attempt to get him to relax a bit more, she smirked at him.

"Why? Did he finally get tired of your lazybones attitude?" This time she definitely didn't miss the way he winced at her joking question, stepping back slightly as if to avoid the topic.

"u-uh...heh...yeah...f-fin-ally, right?" He attempted to pun, his strained smile not reaching his eyelights as she frowned. Since when had he ever struggled to tell a pun?

Deciding to cut to the chase, she dropped her posture, allowing her hands to fall back to her sides. "What do you need, Sans?"

The skeleton paused as he hunched his shoulders, letting his painful pseudo-grin drop as he said quietly, "i-i know it's a lot to ask of ya...but do you think you could let paps into the guard?"

Oh. Oh...so that was what this was about. Of course. Sans' initial hesitance and quiet facadé was just a way of trying to get his brother into the guard. Man, those brothers loved each other way too much. It was sickeningly adorable.

Her grin instantly fell as she became serious. "Look, Sans. Your bro's got the right attitude and the magic to back it up. But I just don't think he's cut out to be a member."

Sans seemed desperate as he argued. "b-but why? please, undyne, he's ready! just let him in already!" The monster's eyelights were flickering a incandescent cerulean and pale gold, lighting up the room in eerie shadows and a powerful, pulsating glow.

Undyne shook her head apologetically, patting the skeleton on the back and ignoring how he became deathly (hah, he'd appreciate that joke) still in favour of reassuring him.

"Sans. I'm sorry, but that's my final say. Break it to Papyrus or not, it's your choice, but he just doesn't have what it takes. He's way too innocent and naive-"

She was interrupted by Sans' soft cries. He was shaking, desperately trying to keep calm as sobs wracked his small frame. "no-no-p-please, if you d-don't let h-him in i'm going t-to g-et such a b-bad beating-"

Sans was hyperventilating in front of her? After unfreezing from her momentary shock, Undyne touched his shoulder as he pushed her back harshly, sending her back a few paces.

"Sans-" She was cut off by the familiar crackling of magic as Sans literally disappeared. Undyne was left standing in her house, not exactly knowing or understanding what had happened, but left with a feeling of guilt and regret for some reason.

An ominous feeling stirred in her gut, one she didn't recognise but made her wary of the future. She'd ask Papyrus about Sans tomorrow. It would be okay.