Undyne’s busy when sans shows up to the playground.
She’s spent the last twenty minutes diligently pumping her skinny little legs on the swingset, aiming for the highest possible point at which she can fling herself off. “Bet you can’t,” Alphys had sneered that morning in class, eyeing Undyne’s gawky frame critically. “You’d break both your arms.”
She might be right but that’s a risk Undyne’s willing to take out of sheer spite, because for some reason, when Alphys laughs at her, it makes her even angrier than when the other kids do it. She’s ready, she’s fully prepared to snap some bones in pursuit of proving her point when, from behind her—
“H-hey! Put her down, you can’t—you can’t do that!”
Undyne smacks herself in the face with her own ponytail, she turns so quick. It’s well worth it—she whirls around just in time to see Papyrus’s older brother, the one who shivers so nervously all the time, and can’t ever quite meet her eyes, pick up Becky, a little brat of a rabbit monster maybe three years Undyne’s elder, by the scruff of her neck. He doesn’t do it gently.
“Sans? What are you—?” She drags the heels of her boots into the dirt, grinding to a stop, and hops off the swing.
“h-hey, Undyne,” he says easily, as though he’s commenting on the weather, or an interestingly-shaped rock. Like this isn’t tilting her whole limited worldview, seeing him outside the confines of their house unaccompanied. It’s weird, sort of like watching a houseplant take itself for a walk, Undyne thinks, blinking wide-eyed up at him. He barely even talks, those few times she’s been allowed over to Papyrus’ house for a playdate.
If he notices her staring, he doesn’t comment. “this is the—the little shit that blacked Pap’s eye, huh? fr-friend of yours?”
He shakes Becky once, vicious. He’s smiling—when is he not—but his left eye is glowing in a way she’s never seen before, this roiling pulse of bright reds and oil-slick blacks. It makes her a little bit sick to look at.
Becky’s sister sniffs wetly from where she’s huddled in the sand, huge fat tears rolling down her cheeks, ears pinned back. “She didn’t mean to!” the kit protests. “He was, he was shouting at her, and he’s always so loud! She just gets mad sometimes, she—“
Sans must not like that answer very much because he shakes Becky again. “c-cool,” he says. “so this’ll be a nice lesson on r-restraint for you, right? right. that’s, that’s a good life skill to learn, kid, you should really be th-thanking me.”
Becky kicks out at him, sneers when her feet don’t connect, “Oh, whatever. What’re you gonna do? You can’t even get through a whole sentence without tripping on it.” She shares none of her sister’s apparent terrified stillness, twisting in sans’ grip like an eel. “Lemme go, come on!”
Thing is, Becky’s a bitch. Thing is, she’s got a bad habit of pulling Undyne’s pigtails when she gets upset, and that’s sort of the worst characteristic Undyne can imagine, so she doesn’t exactly protest when sans’ bloody-black magic seizes Becky tight around the ribcage and hurls her ten feet into the air with no warning at all.
He doesn’t drop her. He throws her bodily to the ground instead, magic crackling like the static on a busted television. She yelps like a kicked puppy, skids to a stop in the sand almost at Undyne’s feet. Undyne doesn’t help her up. She does, however, CHECK her nearly on autopilot, half without meaning to—and her eyes widen when she watches Becky’s HP shudder down a few points. She’s never seen anyone get hurt before, not like that. Not for real.
“let me be abundantly clear,” sans is saying now, letting his eyelights flicker out in that way Undyne’s always found deeply unsettling. He crouches down on his haunches—he’s barefoot, Undyne abruptly realizes, barefoot in the snow and how had she never noticed how weird his feet were before?—just enough to bring himself nearly socket-to-eyeball with the kid, who has somehow managed to pale beneath her honey-brown fur, “you’re not just going to apologize to my brother. you’re going to worship him. the only thing out of your mouth to him until you graduate had better be ‘yes, Papyrus, you’re so fuckin’ great, Papyrus’ or i’ll be back to braid your guts into a goddamn daisy chain for him to wear to the g o d d a m n s p r i n g d a n c e.” He tilts his head, birdlike. “did i stutter that time, you little maggot?”
The girl’s got guts, Undyne will give her that. Sans isn’t big, but he’s a damn sight bigger than a ten-year-old kit, especially one crumpled like a ragdoll in the dirt. Still, she glares up at him with only the barest quiver to her lip, hissing through her tiny sharp white teeth. “You—you can’t do that, we’re kids!” she protests shrilly. “You can’t kill a kid, it’s against the law for a grown monster to kill a kid!”
Sans laughs and stands back upright, his spine cracking and popping with the motion. It’s not a terribly pleasant sound.
“sure. you’re right. that is the law for monsters, isn’t it.” He jams his hands into his hoodie pockets and shrugs. He turns then to Undyne, who’s got her tiny little fingers balled into pale-blue fists, eyes wide, all her jagged teeth bared in excitement at the prospect of watching sans throw down with one of her classmates—especially one that’s worked so hard to get on Undyne’s bad side. “you wanna tell ‘em, little miss fish?”
It takes her a moment to process what he’s asking. Tell her? Tell her what? Tell her Papyrus’s dad keeps a half-mad guard dog on a very thin leash, and said guard dog doesn’t take very well to his charge getting pushed around? Tell her how badly she’s fucked up? Tell her—
Absently, sans is worrying the brass ring on his collar between a cracked thumb and foreclaw. She’s never actually seen him wear it outside the house before either, now that she thinks of it, because—
“He’s not a monster,” Undyne breathes, awed. She’s staring up at him, starry-eyed, impressed, and Becky looks like she’s about to be sick. Sans winks at her.
“funny thing, huh,” he deadpans. “gaster’ll just put me down, write it off as collateral damage…and the next bodyguard probably won’t be so polite, you feel me?”
Becky nods hard enough that Undyne can hear her ears flopping with the motion. “Yes, uh, yes, sir, I hear you, I won’t—I won’t pick on Papyrus anymore, I promise.”
“oh, no. no, honey, that’s not what i asked you for, is it?” Sans rocks back on his heels, looking pleased with himself. “you’re gonna be his best fucking friends, all of you,” he growls. “or we’re gonna see what you look like turned inside out, yeah?”
Undyne nods enthusiastically. She already is friends with the younger skeletons, practically besties—she likes Papyrus and his quick temper and his sheer stubborn rage at a world that refuses to conform to his demands, but hell—
—who is she to argue with that? Sans spits out every words like he’s got power crackling between his teeth, like he’s chewing lightning heedless of the way it burns. It’s a little unnerving, a little unhinged, okay, because realistically, he’s grown and Becky is still a kid, but.
But he’d do anything for Papyrus. Papyrus could make him do anything.