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My Sister Summoned Sirens

Summary:

After registering for an a diving activity, Belle experiences an unforeseen disaster which puts her in the hands of two very dangerous beings to which she manages to make a deal for survival but at a great cost.

Chapter 1: Diving Incident

Chapter Text

“How is— …grrgggg—”

Static clawed through the line, sharp and sudden, cutting Belle off mid-sentence.

“Hello? Wise?” Belle muttered into her phone, frowning as she pulled it slightly away from her ear. She set her scuba mask down on the nearby seat, droplets of seawater sliding across the glass and pooling along the worn leather. The boat rocked beneath her feet, a long, uneasy sway that made the metal walls creak.

For a moment, there was nothing but the low hum of interference and the distant groan of the ocean outside.

Then finally—

“How is the boat? Is the weather okay?” Wise’s voice came through, clearer this time, though still threaded with faint static.

Belle exhaled slowly, some of the tension easing from her shoulders. “It’s… manageable,” she replied, glancing toward the small oval window beside her. Outside, gray waves rolled endlessly under a sky so thick with clouds it felt like the world had been sealed off. The horizon was barely visible, swallowed by mist. “A little rough, but nothing the crew can’t handle.”

“You don’t sound convinced,” Wise said gently, though the worry in his voice was unmistakable. “Belle… it doesn’t have to be today. You can call it off. No one’s going to blame you.”

Belle picked up her mask again, turning it slowly in her hands. The faint reflection of her face stared back at her—tired, but quietly determined.

“It’s not like that,” she said. “I just wanted a simple dive, remember? Clear my head a bit. You’re the one who said I’ve been cooped up too long.”

“I meant getting some fresh air,” Wise replied quickly, his concern immediately surfacing. “Not heading out into rough water like this. You didn’t need to push yourself this far.” He hesitated, then added more quietly, “I just don’t like the thought of something happening and me not being there.”

“Come on bro, you’re being such a tool. It’s just a one-time thing”

“One time thing in what sense?” Wise asked

“Bro, just chill. I’m just having fun for today and I’ll be back by like 6”

“….” There was a silence on the phone

Belle’s expression softened slightly. “Hey… I won’t be alone. There’s a whole crew here and Venus is a dope instructor, I’ll be cool” she said, though her eyes drifted back to the restless sea. “And I’ve handled worse conditions than this.”

A crackle flickered across the line.

“Still,” Wise said, quieter now, more serious. “Rough water, limited visibility—it only takes one thing going wrong. Equipment failure, a bad current…” He stopped himself with a soft sigh. “Just promise me you’ll be careful. Stay shallow if it gets worse. Don’t push it.”

Belle adjusted the strap of her mask between her fingers, feeling the familiar, grounding texture of the material. “I will,” she said. “It’s just a dive, Wise. Nothing extreme. In, out, and back before you know it.”

Silence lingered for a moment.

“…I wish I believed that as easily as you do,” he murmured.

A faint smile touched her lips. “You worry too much.”

“And you don’t worry enough,” he replied, though there was warmth beneath it.

As if in response, the boat lurched violently.

A deep, hollow thud echoed through the hull—too heavy, too deliberate to be just another wave.

Belle stumbled, catching herself against the seat. The mask slipped slightly in her hands.

“Belle?” Wise’s voice sharpened instantly. “What was that? Are you okay?”

She swallowed, heart beginning to race as her eyes locked onto the hatch.

“Just the waves,” she said, though the words felt thin, unconvincing even to her own ears.

Another, softer impact followed—closer this time.

Belle’s breath hitched.

“…At least, I think it is.”

“You think?” Wise deadpanned over the phone

“See you at 6 bro”

“Don’t do anything stupid while you’re—”

CLICK!

“Ohh dear,” Belle sighs.

“Your brother really does care about you,” Venus spoke from the background.

“Yeah… a little too much sometimes,” Belle muttered, walking over to a row of metal storage lockers.

She pulled one open with a sharp clang, hesitating for just a fraction of a second before placing her phone inside. The door shut with a hollow echo that felt louder than it should have.

Venus didn’t comment. She was already by the gear rack.

“Suit up,” she said evenly. “We don’t have time to waste.”

Belle exhaled quietly and moved to comply, her confidence just a touch less solid now. She tugged the suit on, fingers fumbling slightly at the seals before tightening them.

Venus noticed immediately. “Slower. If you rush, you miss things.”

Belle steadied her hands. “I’ve got it.”

“This isn’t about ‘having it,’” Venus replied. “It’s about doing it right.”

She stepped in without waiting, checking Belle’s gear—tightening a latch, adjusting a strap with efficient precision.

“Mask.”

Belle lifted it into place, her breathing sounding loud and unnatural inside the enclosure.

“That’s normal,” Venus added calmly. “First dive always feels wrong.”

Belle let out a small, nervous breath. “Good to know.”

Venus met her eyes through the visor, her voice sharpening with authority.

“Stay close. At all times. You do not wander. You do not improvise. You follow me.”

“…Got it.”

“Say it properly.”

Belle straightened slightly. “I follow you. Stay close. No improvising.”

“Good.”

Venus moved to the hatch, gripping the release lever. The metal groaned as pressure shifted.

A thin line of water seeped through the seam. Belle’s pulse spiked as she stared at it.

“Steady breathing,” Venus instructed. “In. Out. Don’t fight it.”

The hatch lifted.

The ocean surged in—dark, cold, overwhelming—filling the chamber in seconds.

Belle instinctively stepped back.

“Eyes on me,” Venus said firmly.

Belle forced her gaze up, locking onto her.

“On my mark,” Venus continued. “Three… two… one—go.”

Venus pushed off cleanly into the water.

Belle hesitated for just half a heartbeat—

Then followed her through the hatch

—and into the endless sea.

For a moment, everything felt wrong.

The weight of the water pressed in from all sides, the silence broken only by the steady rhythm of Belle’s breathing—loud, too loud. Her movements were slower than she expected, each motion dragging slightly as if the ocean itself resisted her.

She turned her head, and the sight hit her all at once.

An enormous stretch of deep blue surrounded them, fading into darkness the farther it went. Shafts of pale light filtered down from far above, dissolving into shadow long before they reached the ocean floor. The structure they had exited loomed behind them—metallic, artificial, out of place against the vast, shifting expanse.

Belle’s chest tightened.

There’s nothing to stand on.

Nothing to hold.

Her breathing quickened.

Venus noticed instantly.

She moved closer, steady and controlled, placing herself directly in Belle’s line of sight. One hand lifted, palm out—a clear signal.

Focus.

Belle tried, but her eyes flicked past her, to the empty blue.

Venus reached forward, gripping Belle’s shoulder—not hard, but firm enough to ground her.

“Breathe,” her voice came through the comm, calm and unwavering. “Slow it down.”

Belle swallowed, forcing herself to listen.

“In… and out,” Venus continued. “Match me.”

Venus exaggerated the rhythm, slow and deliberate.

Belle followed—shaky at first.

In.

Out.

In…

Out…

Gradually, the tightness in her chest eased. The sound of her breathing softened, becoming less frantic, more controlled.

“That’s it,” Venus said. “You’re fine.”

Belle gave a small nod, still clutching at that fragile sense of calm.

Her gaze drifted again—but this time, slower.

She began to notice more.

Small shapes flickered in the distance—fish, moving as one, their bodies catching the faint light like scattered sparks. Far below, dark formations hinted at rock or coral, their outlines blurred by the depth. The water itself seemed alive, shifting and endless, but not quite as hostile as it had felt moments ago.

“It’s… bigger than I thought,” Belle admitted quietly.

Venus’s posture relaxed, just slightly. “It always is.”

Belle let out a breath that almost sounded like a laugh. “And here I thought I was ready.”

“You are ready,” Venus corrected, her tone firm but steadying. “You’re just new.”

Belle looked back at her.

“Stay close,” Venus reminded her. “Keep your movements controlled. The ocean isn’t your enemy—but it doesn’t forgive carelessness.”

“…Right.”

Venus gave a short nod, then turned, gesturing forward into the blue.

“Follow me.”

This time, when Belle pushed forward—

—she didn’t hesitate.

And together, they moved deeper into the sea.

At first, Belle kept her movements cautious, careful not to drift too far from Venus. But as the seconds stretched into minutes, something began to change.

The fear didn’t disappear—

—but it loosened.

Her breathing settled into a steady rhythm, no longer loud and panicked, but calm… almost natural.

She gave a small kick forward.

Then another.

This time, she didn’t feel like she was fighting the water. Instead, it carried her—slow, smooth, weightless.

Belle turned slightly, more deliberately now, watching as a small school of silvery fish curved around them like a living ribbon. Their bodies flickered in the filtered light, scattering reflections across her visor.

“…Okay,” she said, a hint of wonder slipping into her voice. “That’s… actually kind of amazing.”

Venus glanced back at her, catching the shift instantly.

“Better?”

Belle nodded, a faint grin forming. “Yeah. I think I’m starting to get it.”

Her movements became more fluid—less rigid, less hesitant. She adjusted her position instinctively this time, maintaining her balance without overthinking every motion.

Venus slowed slightly, letting Belle come level with her.

“Good,” she said. “That’s what I want to see.”

Belle let out a quiet laugh. “Would’ve been nice if my nerves got the memo before I paid for this.”

Venus’s tone softened—just slightly.

“You didn’t pay to panic,” she said. “You paid to enjoy it.”

Belle glanced around again.

The vast blue didn’t feel quite so suffocating now. The light filtering down from above danced gently through the water, and the distant shapes of rock and coral seemed less like shadows… and more like something waiting to be explored.

“…Fair point,” Belle admitted.

Venus gestured outward. “So enjoy it.”

Belle took a moment—just one—to really look.

The slow drift of the water.

The quiet elegance of the fish.

The way everything moved without hurry, without noise.

A small, genuine smile spread across her face.

“Okay,” she said, more confident now. “Yeah… I am.”

Venus gave a short, approving nod.

“Stay close,” she reminded her. “But relax into it. The water works with you, not against you.”

Belle adjusted her movement, gliding forward with a touch of ease now—still careful, still aware, but no longer tense.

And as they moved through the open water—

the ocean didn’t feel overwhelming anymore.

It felt… freeing.

For a few quiet moments, everything was calm.

Then—

Something shifted.

At first, it was subtle. A faint pull beneath them, like a current changing direction. The water, once smooth and weightless, began to twist.

Belle frowned, adjusting her position.

“Venus…?”

Venus had already noticed. Her head snapped toward the distance, body going rigid.

“Hold position,” she commanded, sharper now.

The water ahead of them darkened—

then began to spiral.

A slow, unnatural rotation, growing faster by the second. The light above bent strangely as the current tightened into itself, forming a massive, churning vortex in the water.

Belle’s eyes widened.

“That’s… not normal, right?”

“No,” Venus said immediately. “It’s not.”

The pull intensified.

Belle felt it tugging at her legs first, then her entire body—dragging her toward the spinning void.

“Stay with me!” Venus shouted, swimming toward her.

Belle kicked hard, trying to fight it. But the closer the vortex grew, the stronger the force became—grabbing hold of her like invisible hands.

“I—I can’t—!”

“Don’t fight it blindly!” Venus ordered. “Angle your—”

The current surged.

It hit Belle like a wall.

Her body spun violently, orientation lost in an instant—up, down, left, right all dissolving into chaos. The blue world around her twisted into streaks of light and shadow.

“Belle!”

Venus’s voice cracked through the comm—but it was already starting to distort.

Belle reached out, her gloved hand stretching toward her—

—but the distance between them snapped wide open in seconds.

“No—!”

The vortex yanked harder.

Venus fought against it, straining, arm extended—but the force tore Belle away completely, pulling her down into the spinning darkness.

“BELLE, STAY WITH ME!” Venus’s voice shouted—loud, urgent—

then fainter.

“Don’t—lose—focus—!”

Fainter still.

“Belle—!”

The sound broke apart into static.

The world spun faster.

Belle couldn’t tell which way was up anymore. Her breathing became sharp, uneven—too fast again. Her chest tightened as panic clawed its way back in, stronger than before.

The light above vanished.

Cold darkness swallowed everything.

I can’t—

Her vision blurred.

Her body felt heavy now… too heavy to move against the current dragging her deeper… deeper…

The last thing she thought of—

was her brother.

His voice.

His warning.

Don’t do anything stupid…

A weak, almost ironic smile flickered across her face—

Then everything went black.

 

Darkness lingered.

Then—

A sound.

Faint at first.

Voices.

Soft… distant… female.

“…you sure she’s okay?”

“I don’t know how surface monkeys work, I’m guessing she might be dead”

Belle’s mind stirred, sluggish and heavy, like it didn’t quite belong to her yet. The voices blurred together, slipping in and out of reach as if carried on water.

Her vision flickered.

Darkness gave way to haze—shapes without meaning, shadows without form.

She tried to move.

Nothing.

A dull awareness crept in instead.

Cold.

Not the crushing, suffocating cold of the ocean—but something different. Damp. Still.

Solid.

Her back was flat against something hard.

Stone.

The realization came slowly, drifting into place as her senses sharpened.

Her fingers twitched.

Her chest rose—air filling her lungs freely, without resistance.

Air…?

Her eyes forced themselves open.

At first, everything was blurred—light and shadow bleeding into each other. But gradually, the world began to take shape.

Above her—

Jagged formations stretched down from the ceiling.

Stalactites.

Dark, wet stone glistened faintly, catching what little light there was. Drops of water clung to their tips, occasionally falling… the soft, echoing drip cutting through the quiet.

The space was dimly lit—no clear source, just a low, ambient glow that made the cavern feel both visible… and endlessly shadowed at the same time.

Belle’s breathing hitched slightly as awareness fully returned.

Her mask—

Gone.

Cool air touched her face, damp and heavy with the scent of stone and water.

“…Hey—her eyes are open.”

The voice was closer now. Clearer.

Belle’s gaze struggled to shift, dragging itself away from the ceiling above.

Her body still felt weak, distant—like she hadn’t fully returned to it yet.

The last thing she remembered—

Water.

The vortex.

Venus—

Her chest tightened slightly at the thought.

But the ocean…

was gone.

And somehow—

She wasn’t.

In a matter of moments, she felt a strange pressure building in her chest—tight, suffocating—until it forced its way out.

COUGH! COUGH!

Belle doubled slightly, raising a trembling fist to her mouth as she hacked violently.

Water—actual water—spattered against her knuckles, each cough dragging more of it from her lungs.

Her breath came in ragged bursts.

“Easy…” a voice said, almost lazily amused. “She’s purging the last of it.”

Belle barely registered the words. Her body shook with each cough until, finally—

it stopped.

She inhaled sharply.

Air.

“Oh look,” another voice chimed, lighter, almost sing-song, “the surface monkey is up.”

“Huh?” Belle muttered weakly, her voice hoarse and strained.

Slowly—carefully—she turned her head to the right.

And froze.

Right in front of her were two peculiar beings.

At first glance, they looked like women—beautiful, almost unnaturally so. One had long blonde hair that shimmered faintly in the dim light, her expression still and unreadable. The other’s black hair flowed in loose waves, framing a face that carried a subtle, mischievous smile.

But then—

her eyes dropped.

The blonde one’s skin was lined with fine yellow scales, catching what little light there was, while the dark-haired one’s red scales gleamed richer, deeper, almost warm against the shadows.

And below their waists—

no legs.

Only long, powerful, fishlike tails, shifting slowly against the damp stone.

“…..”

Belle’s mouth parted—

but nothing came out.

Her brain stalled completely.

The blonde one watched her in silence, gaze sharp, detached, almost clinical—as if studying something far less important than a person.

The black-haired one, on the other hand, tilted her head, clearly entertained.

“AHHHHHHHHHHHHHH!” Belle screams extremely loudly as she tried to crawl away but could not get that far as the rocks were too slippery for any meaningful movement

“Oh, she’s really trying to process it,” she said, a quiet laugh in her voice. “That’s adorable.”

“Enough,” the blonde one said flatly. Her eyes never left Belle. “You’ll overwhelm her.”

“I’m not doing anything,” the black-haired one replied lightly. “She’s overwhelming herself just fine.”

Belle swallowed hard, forcing her voice through the fog in her head.

“…I’m not—” her voice cracked. She tried again. “…fragile.”

That got a reaction.

The black-haired one smiled wider. “Oh, it talks back too. I like her.”

The blonde one didn’t smile at all. If anything, her expression grew colder.

“Can it stand?” she asked, not even addressing Belle directly.

“Well, it has to, right? I mean, surface monkeys do move on two legs,” the black-haired one said, glancing over at her companion.

“But this one is a female. We’ve only killed males—it could be entirely different biology,” the blonde one replied.

‘Killed…’

Belle’s heart dropped into her stomach the moment she heard that word.

“Hey, can you stand like the male surface monkeys?” the black-haired one said, turning her attention back to Belle.

Belle didn’t move. She stayed frozen on the ground, paralyzed with fear.

“I don’t think it can talk much,” the black-haired one whispered.

“P-Please…” Belle muttered.

“Oh, I was wrong,” the dark-haired creature corrected quickly.

“P-Please, h-have mercy. I-I didn’t want to bother anything or anybody—I was just swept in here by the vortex by accident,” Belle pleaded, clasping her hands together.

The two creatures exchanged a glance before looking back at her.

“You’re telling us information we already know,” the blonde one said flatly.

“W-What?” Belle stammered.

“I’m the one who brought you here. Do you really think I don’t know who I take?” the black-haired one said, causing Belle’s face to go pale.

“Y-You…”

“Though I didn’t manage to grab the other one,” she added, rubbing her chin thoughtfully as she looked upward.

“You were too lazy. You could have caught her if you’d put in a bit more effort,” the blonde one cut in.

“Shush, Evelyn—not in front of the food,” the black-haired one said.

“It’s just food, Astra,” Evelyn replied.

“FOOD!?”

Belle scrambled desperately to her feet, but the slick stone caused her footing to fail. She slipped and crashed back down. Despite the impact, she barely felt any pain—her adrenaline had completely taken over.

“Of course you’re food. How else is a siren supposed to eat? Duh,” Astra said, raising an eyebrow.

“Come on, can’t we just… not eat me? I mean, there’s a whole ocean full of fish—why humans?” Belle said with a strained, nervous smile, sweat forming across her brow.

“She’s just like the others we’ve killed. Nothing original,” Astra sighed, her posture slumping in disappointment.

“W-What?” Belle blinked, glancing between them.

“I agree. This one is just as uneventful as the other humans,” Evelyn nodded.

“And to make it worse, they just dive into the water themselves,” Astra added with a pout. “I didn’t even have to sing or lure them. They practically jumped onto our plate. Nothing interesting today—just a dead catch.”

“They never say or do anything interesting,” Astra continued, dragging a hand down her face. “Nothing that makes them stand out. It’s always the same: ‘Don’t kill me,’ ‘Spare my life.’ Nobody ever does anything different.”

A brief silence settled over the cavern before the two sirens turned their attention back to Belle.

She tensed.

“Any last words?” Astra asked casually.

‘Shit… they’re going to kill me.’

‘I can’t believe this… dying to something I didn’t even think was real…’

‘They won’t listen… everyone else begged too…’

Belle swallowed the lump in her throat.

“So how are we eating this one? You’re taking the legs this time, right?” Astra asked.

“No. I want the arms,” Evelyn replied.

Astra sighed. “This is such a drag. This one looked interesting too…”

‘Interesting…’

Belle repeated the word in her mind.

‘They said I was boring… wait… maybe I could—’

“H-Hold on,” Belle said quickly.

“Just save it. We already know you’re going to beg. Sit there quietly—I’ll make it quick,” Evelyn said coldly.

“I-I can do something for you!” Belle blurted out.

“No thank you. Plenty of offers like that. Most of the others couldn’t dance either,” Astra said dismissively.

“N-Not dancing! I—I can give you something… as long as you spare me,” Belle said, clasping her hands again.

Astra and Evelyn exchanged another look before turning back to her.

“You have something original?” Astra asked.

“I-I think so?” Belle replied timidly.

The two sirens looked at each other once more. Evelyn let out a quiet breath, while Astra gave a small shrug.

“…Okay,” Astra said at last. “Let’s hear it.”

 

------------ 3 HOURS LATER -----------

 

“And uh… yeah. That’s everything.”

Belle finished, her voice trailing off as the lump in her throat refused to go away. Her hands stayed clasped tightly in front of her, fingers fidgeting slightly. She didn’t dare look up—her eyes remained glued to the floor as if it were the only stable thing left in the room.

The faint ticking of a wall clock filled the silence.

“…..”

Wise said nothing.

A thin line of sweat rolled down the side of his face, tracing along his jaw before disappearing beneath his collar. He stood stiffly, shoulders tense, his gaze fixed deliberately off to the side—anywhere but the two figures sitting on his couch.

They were in his house.

On his couch.

…and he was actively pretending not to see them.

“Well explained, human woman,” Astra said, her tone bright and approving, a soft clap of her hands breaking the suffocating silence.

The sound echoed just a little too loudly in the quiet room.

“…T-Thanks…” Belle murmured, barely audible.

“So,” Evelyn said, her voice cutting cleanly through the air, “you understand your situation now?”

Her eyes locked onto Wise—unblinking, cold, observant.

Wise flinched.

“M-Mhm.”

He nodded quickly—too quickly—his lips pressed into a tight, almost painful line.

“Good,” Astra chimed, her voice lightening the atmosphere in a way that somehow made it worse. “Then we can get comfortable, it seems.”

With casual ease, she leaned back into the couch and kicked her feet up onto Wise’s coffee table—completely unbothered by the space she was invading.

Her movements were relaxed. Confident.

Natural.

As if she belonged there more than he did.

Wise’s eyes twitched slightly—but he didn’t say anything.

He couldn’t.

Astra tilted her head, catching that reaction instantly.

Her gaze locked onto him.

“You don’t mind much, do you?” she asked, her tone playful—teasing, even—but her eyes watched him very closely.

Wise swallowed hard.

“U-Uhm…”

He brought a fist up to his mouth, clearing his throat quietly, buying himself half a second to compose something—anything—coherent.

“I—I don’t,” he said, his voice low and tight.

Astra’s smile widened just slightly.

“Good.”

“Not like you have a choice,” Evelyn added flatly.

The words landed heavy.

‘Sirens… s-sirens exist…’

Wise’s thoughts spiraled violently, refusing to settle.

He risked a glance.

Just one.

His eyes flicked toward Astra—

—and immediately regretted it.

She was still looking at him.

Still smiling.

Like she’d been waiting for him to look.

His gaze snapped downward again, locking onto his hands resting in his lap.

They were clasped tightly. Too tightly. His knuckles were pale.

‘And we have to entertain them…’

His fingers trembled slightly, barely noticeable.

His chest felt tight.

Heavy.

‘…or die.’