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English
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Published:
2026-08-17
Updated:
2026-08-17
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2,548
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1/?
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Wonder Girl and the Slayer

Summary:

Traditionally, Slayers are supposed to work and die alone.

Unfortunately for The Powers That Be, neither Buffy Summers or Cassie Sandsmark have been the type to blindly follow the rules.

Notes:

(See the end of the work for notes.)

Chapter Text

Cassie’s choppy blonde hair fluttered in the breeze as her skateboard clattered down the quiet suburban street, her mind locked on one thing; confronting one Buffy Summer. She knew they weren’t exactly on the best terms these days, what with Buffy’s other friends having some weird thing against her, but Buffy still personally invited Cassie to her birthday party a few weeks back. That definitely had to count for something, right? Then again, Cassie figured Joyce made Buffy invite her.

She skids to a stop just before she hits the creaky porch stairs, kicking up her board and tucking it beneath her arm before knocking on the door, heavy-handed. Cassie whistles a jaunty off-key tune as she waits, looking around the block and noticing that Joyce’s car wasn’t in the driveway. It made her think that maybe she should’ve called ahead of time before dropping in. Considering Buffy’s go-to excuse these days was being ‘busy’, it’d be just her luck that she’d be gone this time too. Ugh.

Just when Cassie was about to cut her losses and head right back home, the door slams wide open with a bang. Standing in the frame was Buffy's little sister, Dawn. Her long brown hair was tied into two loose braids, and her startled was face smudged with something purple and glittery. She tried to hide a Barbie doll behind her back, but Cassie immediately noticed it was coated in that same glittery stuff as her face. Cassie grins, casually leaning against the porch post behind her.

“Hey, Dawnie! What’s with the…?” Cassie trails off, waving a hand around her own face. Dawn shrugs, smiling back as she rocks back and forth on her feet.

“I borrowed Buffy’s eyeshadow. She hasn’t noticed yet, so don’t tell her.”

“Then you’d better not let her see you walking around like that,” Cassie says, slightly thrown off, “Um. Speaking of Buffy, is she around? I really gotta talk to her. Super important friend business. It’s on a strict need-to-know basis.”

Dawn’s nose wrinkles,“Yeah, she’s here, but she’s been really grumpy and mean all morning. I don’t think you should talk to her.”

“Oh yeah?” That struck her as especially weird. Mean was a toss-up when it came to Buffy, but Cassie couldn’t really think of a time where she was mad enough to take it out on Dawn. That just wasn’t her style.

“Yeah,” Dawn rolls her eyes, “She barely left her room, and mom’s mad at her because she came in super late last night and wouldn’t say where she was. That’s not even the first time she’s dones that.”

“Well, if she’s not talking to anyone else, maybe I could get through to her. I’m her best friend, y’know?”

Dawn frowns, dragging a toe against the ground, “I dunno, Cassie. I’m not really supposed to let people in when mom’s at work. I’m not even supposed to let my friends in.”

“We could just tell her I snuck in past you. Oh! Or I can just leave before she gets back!”

“But won’t you get in trouble if she catches you anyway?”

“Aww, trouble-schmouble!” Cassie waves it off with a dismissive snort, “Joyce loves me. She’d probably let me in anyway if she was here.”

“Okay… but if Buffy hurts your feelings, don’t say I didn’t warn you.” Dawn hums in a sing-songy way, stepping to the side to let her in. Cassie affectionally ruffles her hair with another toothy grin, before loudly running up the stairs.

“Buffy! Buffster! Buffs!” Cassie hollers the moment she hits the top landing. Practically growing up with the Summerses meant that she could walk the entire layout of this house blindfolded, and still manage not to run into anything. Plus, she remembered where everyone’s rooms were! She pads down the hall, stopping in front of a door covered in little fake gems and flower stickers. Cassie doesn’t bother to stopping to knock.

When she barges in, Cassie’s bombarded with the scent of some yummy-smelling raspberry body splash. Sprawled across her bed like a lazy cat was Buffy herself, her eyes shut. Sleek headphones covered her ears and her CD player rested against her stomach as she mouthed the words to what Cassie assumed was some pop song.

As rude as it was to disturb her peace, Cassie didn’t really care. She props her skateboard against the wall by the door, crossing the obnoxiously pink shag carpet that covered most of Buffy’s floor, and with a mischievous smirk, she reaches forward and grabs Buffy’s ankle.

“What are you listening to?!” She growls in an attempt to sound scary.

Buffy tugs off her headphones with a startled yelp, shooting up into a sitting position and messing up her perfectly flat-ironed hair in the process.

“Oh my god, Cassie! You almost gave me a heart attack! When did you—did Dawn let you in?”

“Pssh, no,” Cassie immediately lies, shoving her hands into the deep pockets of her jorts, rocking back on her heels. “Your mom gave me a spare key that one time! Duh.”

“I’m sure she did,” Buffy eyes her skeptically, before swinging her legs over the bed, “What’re you doing here anyway? Don’t get me wrong, mom says you’re always welcome, but you didn’t even call.”

“I can’t believe you’re asking me that.” Cassie folds her arms now, her easygoing attitude quickly giving way to utter frustration. “See, a few weeks ago, I remember you saying you wanted to start hanging with me again. But then you went back to avoiding me like, a week later. I wanted to know why.”

“I’m not trying to avoid you. I’ve just been super busy lately. Like, unbelievably busy! So busy, in fact, that it’d probably take forever to explain.”

“Oh, like I haven’t heard that one before,” Cassie rolls her eyes, turning her attention toward Buffy’s cluttered vanity set, picking up a teetering family photograph, “What’s got you so ‘busy’ anyway? Your other friends? You guys might as well be conjoined.” She says sarcastically, setting the picture down somewhere safer.

“Well… not exactly?” Buffy’s face scrunches awkwardly, her voice pitching upward. She joins Cassie at the table, carefully angling herself to block the other girl’s line of vision from something.

“Then what exactly?” Cassie tries to pry, putting her hands on her hips.

“I can’t tell you.”

“You can’t or you won’t?”

“Can’t.” Buffy insists, inching her hand toward something on the vanity. Cassie’s eyes track the movement. Her own hand darts out with divine speed, and it takes a moment for Buffy to register what just happened. Panic fills her eyes as she tries to snatch it away, grabbing Cassie’s wrist tightly.

“Give it back!” She shrieks, “I mean it, Cassie!”

Cassie pokes out her tongue as she tosses it to her other hand, holding Buffy back with a firm forearm against her sternum. She doesn’t exactly know what she was expecting the mystery thing to be, but it definitely wasn’t—

“A stake?” Cassie raises her eyebrow, bouncing it in her hand so the sharp part safely pointed downward. “You’ve been ignoring me because you secretly got into woodworking?”

With a surge of strength Cassie was not expecting from her much prissier friend, Buffy manages to swipe the sharpened wood back, hugging it to her chest protectively.

“Yes! I mean, no!” Buffy moves to crouch front of a sticker-covered chest at the foot of her bed, hurriedly shoving the stake in through the tiny gap. It was probably the last thing she wanted, but Cassie noticed something silver glinting inside just before Buffy slams the top back shut and sits on it. “I mean… god, Cassie! Just drop it, okay? You got what you came here for, and now you know I’m totally fine.”

“Uh-uh, no way,” Cassie stomps forward, “You’re never this jittery! Not even when you get those gross frappucinos from Starbucks before school.”

“Hey, you take that back; those taste great and you know it!”

“Yeah, yeah, whatever. But that’s not the point! The point is that you’re being weird, and I’m gonna get to the bottom of it, even if I have to camp out in your room all day!”

“If you’re staying that long, that’s gonna be a problem.” Buffy says, drumming her nails against her thigh, “I got plans tonight. Major plans that I so can’t cancel.”

“Dawn says you’ve been coming home late for days now.” Cassie narrows her eyes, “What’re you doing?”

“Aha! So she did let you in!”

“Again, not the point.” Without being asked, Cassie flops down onto the chest next to Buffy with a loud creak. “Just give it to me straight. Are you in trouble? Because if you are, I can totally help!”

“I really love your enthusiasm, but you can’t help me. You probably wouldn’t even get it.” Buffy stares down at her hands clasped in her lap, exhaustion starting to bleed into her words, “And before you ask, my mom can’t either. No one can. Not in a way that’d matter.”

Cassie frowns as she stares at Buffy’s side profile, trying her hardest to figure out what manner of hopeless thoughts were was going through her head right now. Did she get caught up in a bad crowd or something? No, Buffy wasn’t like that.. at least, not the Buffy she grew up with. As conceited as they were, Cassie wouldn’t consider her new friends to be outright evil.

She reaches over to grab her hand, causing to Buffy look up in surprise. Cassie shoots her a crooked smile, gently squeezing her hand.

“Look, I might not ‘get’ whatever’s biting you, but that doesn’t mean I can’t lend an ear. Maybe I’d understand better than you think?”

Buffy stares her down, quiet. It’s a little uncomfortable, actually. She’s usually on the chatty side. Cassie’s tempted to fill the silence with more reassurances, or at least something helpful, but then Buffy nods stiffly and squeezes her hand back.

“Alright… do you remember that new librarian from England we got a few weeks back? Mr. Giles?”

“Yeah, he totally saved my Bio grade last Friday with his textbook recs! What about him?” Cassie asks, tilting her head confusedly. Buffy let out a breath, as if to brace herself.

“So, like, turns out he was sent here for me. Not like, as a tutor or anything. Well, no, he’s sort of a tutor but not for school stuff-” Buffy trails off, clearly frazzled, and then shakes her head, “Okay, okay! So, he’s supposed to teach me how to slay the undead. Apparently there’s this whole, like, super ancient prophecy that says that one girl in every generation gets picked to fight monsters. I’m that girl. I’m what’s called a Slayer.”

Cassie blinks, her jaw dropping at the sudden word vomit. The concept of a Slayer wasn’t too out of the ordinary for Cassie, seeing as she was literally Wonder Girl, but Buffy being one? Buffy who cried about broken nails after gym class and matched her lip glosses with every outfit and couldn’t stand horror movies? No offense, but who in their right mind would make her fight, what? Zombies and vampires? That was just asking for trouble.

“Dude,” Cassie finally manages, breathless, “Does that mean you have superpowers?”

“Something like that,” Buffy responds, flexing her free hand thoughtfully, “According to Giles, I’m stronger, I can run faster, and my reflexes are insane. I still don’t know if that’s everything I can do. I only found out literally days after my birthday.”

Oh. That’d explain the snubbing. Now Cassie felt bad for assuming the worst. She rubs the back of her head before letting go of Buffy.

“I guess you’ve been pretty busy slaying stuff then, huh?”

“As if,” Buffy laughs sharply, “Giles wants me to learn the ‘fundamentals’ first before I actually go out there. Kinda hard to learn weaponry when he’s got the physique of a wet paper bag.” She finally cracks a grin, “I’m surprised you’re taking this so well, Cass. I mean, if this was my mom, she’d wig out!”

“That’s only because I have powers too,” Cassie smirks, “I’m Wonder Girl.”

Buffy eyebrows shoot up, “Shut up. There’s no way!”

“Way!” Cassie hops up from her seat to demonstrate. She looks around the room for a moment, before shrugging and bending down to wrap her arms around the chest, gritting her teeth as she carefully begins to lift it. Of course, she takes great care to make sure Buffy doesn’t slide off the edges.

“Ta-da!” Cassie cheeses, shifting to a one-handed grip to flourish her other. The moment her arm begins to wobble though, she quickly puts the chest back down with a grunt.

“I can’t believe this.” Buffy giggles, standing up too, “You’re Wonder Girl! I’ve been hearing about you on the news for, like, a month straight! Mom’s, like, super worried because Wonder Girl’s so young, but she just doesn’t get it!”

“It’s cool, right?” Cassie grabs Buffy hands again, practically shaking from excitement, “Ooh, idea! What if you did hero stuff too? We could be like the Titans or something!”

“Wait, me as a superhero?”

“Why not? You’ve got the powers, you’re getting the training, and you’re supposed to fight evil stuff anyway! It’d literally be perfect!”

Buffy bites her lip, “I don’t know.. this whole slayer thing’s supposed to be a one-person job.”

“Why?”

“Because that’s just how it’s supposed to be, according to Giles. It’s, like, always been that way.”

“Why can’t we switch it up?” Cassie questions, “I mean.. shouldn’t Slayers be able to get help whenever they need it? You already have a mentor, and that can’t possibly be enough to take on the world! Plus, I have powers too, which means I won’t get in your way at all.”

Buffy considers this, until another small smile crosses her face, “Okay… you might actually have a point. I’d probably have to ask Giles first since he’s technically in charge, but there’s no way he could say no!”

“Even if he does, I’ll so crash your patrols anyway. I’ll make a cool heroic entrance and everything!”

“I’m meeting with him again tonight, so I’ll ask him then.”

“Need me to come along and help you do some serious convincing like how we used to ask for sleepovers?”

“Hmm… nah. I don’t wanna get your hopes up, just in case. I’ll call you right after my slaying lessons so I can tell you how it went though!”

“Deal.” Cassie squeezes her both hands before tugging Buffy along, only pausing to pick up her discarded skateboard.

“Oh, and by the way, d’you mind not telling my mom about this? She’s got this whole thing against superheroes, and I’m already on really thin ice right now. My training runs really late, and I’m trying to get used to pulling all-nighters.”

“Your secret’s safe with me. Scout’s honor.” She says solemnly, giving Buffy a mock salute. Amused, Buffy gives her a playful shove foward.

“You weren’t even a scout! You said you’d never sell a cookie for as long as you lived.”

“It’s the thought that counts.”

All Buffy can do is laugh and shake her head, bumping her shoulder against Cassie’s as they head downstairs together.

Notes:

hi there!! i've been seeing so many buffy/dc crossover fics lately, and i've decided to give it a try myself!
some things to note: this will mostly be following wonder woman '87, young justice '98, and teen titans '03, as well as other post-crisis lore if needed. on the buffy side, i'll be mish-mashing most of the season together! i'm not strictly adhering to anything.
also technically speaking, this would be hemery high era buffy! don't worry about the master.
and lastly, this won't be very favorable towards angel/bangel. i'm sorry.