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Senku and Gen came to an agreement two weeks ago to engage in an arrangement that included making out and, if those kisses resulted in sex—then they’ll have sex.
Let's just say that kissing Senku has become the norm.
What about the sex though? They haven’t done it...
… yet.
***
The first time it happened was two weeks after Gen had recovered from her fever. Winter gripped the kingdom of science tightly, blanketing everything in a relentless chill that seeped through the walls of their hut in the observation room.
To combat the cold, Senku had suggested they cuddle for warmth, a practical solution from the usually touch-averse scientist who clearly despised the freezing temperatures more than he let on.
One morning, Gen stirred awake first—a rarity, since Senku was always the early riser.
She lay with her back pressed against him, his arms wrapped snugly around her waist, his body molded to hers in a way that felt both intimate and insistent.
He had shifted closer during the night, his chest flush against her spine, his hips nestled right up to her ass.
The heat from his form was a welcome contrast to the icy air, but as awareness fully settled in… well… Gen kind of froze.
Because…
Something firm poked between her legs, insistent and throbbing through the thin layers of their clothing.
It’s Senku-chan’s morning wood, she realized with a flush creeping up her neck.
His cock, hard and unyielding, nudged against the cleft of her ass and the warmth of her pussy, separated only by fabric. Heat pooled low in her belly, a mix of surprise and curiosity swirling inside her.
What should she do? Pretend to sleep it off? Slip away quietly? They had talked about it—an arrangement where they could explore sex without complications—but so far, it had only led to making out, nothing more.
Gen's mind raced. Should she wake him? Or just... wait?
As she debated, her body betrayed her. She shifted slightly, just a subtle roll of her hips, and the friction sent a jolt through her.
She gasped softly, feeling the thick length of his dick slide along her clothed pussy, the pressure teasing her hole even through the material.
Senku-chan… he’s definitely big.
And a thought of:
Would that even fit inside me?
Shaking her head to clear it, Gen decided she couldn't act without his consent. He wasn't awake yet, and this was too important to stumble into blindly. She began to ease forward, trying to disentangle herself without disturbing him.
But then Senku stirred behind her, his breathing changing rhythm. That meant he was waking up.
"Mentalist?" His voice was rough with sleep, groggy but alert.
Gen stilled, the air between them thickening as the reality of their position sank in. She heard his breath hitch sharply, felt the subtle tension in his body as he registered the hardness pressed against her. He must have known she was awake now, the game up.
"Shit, mentalist," Senku muttered, his voice laced with embarrassment. His arms loosened, starting to pull away. "I didn't mean—"
She didn't know what possessed her—maybe the ache building between her legs, or the way she didn't want him to apologize for something her own body craved..
Because before Senku could retreat fully, Gen arched back, grinding her ass firmly against his cock. The hard shaft slotted perfectly between her cheeks, rubbing along her entrance through their clothes.
Or maybe Gen was in a mood to be playful.
Senku choked out a sound, half surprise, half groan, his hands freezing mid withdrawal.
That was all the encouragement she needed. Gen rocked her hips again, deliberate this time, pressing down to feel every inch of his erection drag against her. The friction sparked heat through her core, her entrance growing slick beneath her undergarments.
Senku's breath stuttered, and then his arms snapped back around her, one hand splaying across her stomach while the other gripped her hip, holding her in place.
"Gen..." he rasped, but there was no protest in it, only hunger in his tone.
He thrust forward tentatively, his cock grinding up along her ass, the tip bumping her entrance with each pass.
Gen moaned, low and needy, and matched his rhythm, circling her hips to increase the pressure. Their clothes chafed deliciously, the barrier heightening the tease as his dick throbbed against her soaked folds.
The hut filled with the sounds of their shared breaths, ragged and quickening. Senku's grip tightened, his fingers digging into her skin as he rutted harder, sliding his cock between her thighs now, the head catching on her entrance through the fabric. Gen whimpered, grinding back with abandon, her hole pulsing with each slide.
Sweat beaded on their skin despite the winter cold, their bodies heating the small space between them.
It was frantic, messy—two inexperienced parties discovering the raw edge of desire.
Senku's thrusts grew erratic, his cock twitching violently against her.
“Gen.” Senku whimpered.
Then with a guttural groan, he came, hot spurts soaking through his pants and dampening the fabric between her legs.
He shuddered against her, panting harshly, his usual composure shattered as he buried his face in her hair.
Gen slowed her movements, her own arousal simmering but unfulfilled, a delicious ache lingering in her pussy.
She turned her head slightly, watching in fascination as Senku caught his breath, his hair disheveled, cheeks flushed, eyes glazed with satisfaction. He looked like he'd just unlocked a groundbreaking formula, utterly wrecked and thrilled by the discovery.
"That was amazing," Senku murmured, his voice soft and awed, still wrapped around her.
Gen couldn't help but snort in amusement, the sound light amid the heavy air. Seeing the scientist so undone, so blissed out, made her heart—and other parts—flutter with possibility.
Then Senku glanced at Gen, his eyes still hazy from his release.
"Were you able to orgasm?" he asked in a husk voice.
Gen blinked, the question pulling her from the lingering warmth in her core. Right—she hadn't.
That’s when Senku frowned, his brow furrowing in that analytical way of his.
Before she could wave it off and say it was fine, he sat up abruptly, the furs shifting around them.
He loomed over her body, his frame casting a shadow in the dim morning light filtering through the hut's cracks. His left hand hovered above her waist, fingers twitching slightly as he swallowed hard, cheeks flushing a deep red.
"Can I touch you..." He paused, the words sticking in his throat. "... there?"
Gen felt her own face heat up, the implication clear as day. He meant her pussy, the slick ache still throbbing between her legs.
She searched his face, if there’s hesitation—maybe he felt obligated, pushing himself because she hadn't climaxed. But no, his expression held pure determination, laced with the uncertainty of someone stepping into uncharted territory for the first time.
So she nodded, her heart pounding.
Senku exhaled slowly, his hands moving with careful intent. He helped her unwrap the layers of her clothes, peeling back the fabric until her lower half was exposed to the cool air. The chill kissed her skin, but the heat from her arousal made it negligible.
When Gen’s thighs parted slightly, Senku's eyes widened at the sight—her slit was definitely wet as her folds were swollen and slick from their earlier grinding.
"You're soaked," he murmured, almost to himself, fascination edging his voice.
Gen swallowed.
This is kind of embarrassing, Gen thought.
Then Senku’s fingers descended tentatively, the pads of two digits circling the outer lips of her pussy. Gen gasped, the light pressure sending sparks up her spine. He started slowly, tracing the slick contours, spreading her arousal around her entrance without dipping inside.
His gaze flicked between her cunt and her face, switching rapidly like he couldn't decide where to focus, his scientific mind cataloging every detail but avoiding lingering too long on any one spot, as if committing it all to memory without overindulging.
And…
Senku seemed utterly invested, treating her body like a complex mechanism he was reverse-engineering.
Twisting his fingers gently at first, rubbing in small circles over her clit, then sliding down to part her folds and stroke the sensitive inner edges.
Senku murmured, “you're so tight.”
“Senku-chan…” It made Gen arched slightly, a soft moan escaping her lips.
With that Senku adjusted as he pressed firmer on her clit. And then he dragged his fingertips along the seam of her pussy, up and down, building the friction without ever pushing in.
Each movement felt deliberate, realistic in its trial-and-error: a curl here to graze her hole from the side, a flat press there to cover more area, always watching her reactions to refine his approach.
Gen's breaths came quicker, her hips bucking involuntarily as the pleasure coiled tighter. His free hand rested on her thigh, holding her steady, while his focused eyes darted between her face and her hole and absorbing how her muscles tensed, how her gasps turned to whimpers.
The tension snapped suddenly as Gen came with a cry.
Her body floating on waves of ecstasy, every nerve alight as her entrance clenched around him. Pleasure washed over her in pulsing surges, her slit throbbing under his circling fingers, slickness coating his hand.
She trembled, limbs heavy and light all at once, the afterglow settling like a warm blanket—euphoric, boneless, her skin tingling with residual sparks. Colors seemed brighter in the hut, her mind hazy and content, the world narrowing to the soft press of his touch lingering on her oversensitive folds.
Senku withdrew his hand slowly, wiping it on the furs as he watched her come down, his own expression a mix of awe and satisfaction.
Gen sighed, a lazy smile spreading across her face. "If sex is like this, there's no hame-say in being a slut. Actually, everyone hould-say become a slut."
"Ten billion percent," Senku agreed without missing a beat.
She snapped her head toward him, surprised to find him already laying beside her, propped on one elbow with a grin splitting his face.
"Did you eriously-say say that, Senku-chan?" Gen asked, shock lacing her tone amid a bubble of laughter.
"What? It's the truth." His grin widened, unrepentant. "Sex is exhilarating. Let's do further research."
Gen rolled her eyes, but she couldn't deny it—she agreed completely, the spark of curiosity mirroring his own.
Afterward, they cleaned themselves up as best they could with the limited supplies in the hut, wiping away the evidence of their morning with damp cloths and fresh layers of clothing.
With a shared sigh of relief, they descended the stairs to the main area, only to find it empty.
Chrome must have already ventured out, probably scavenging for interesting rocks or chasing some wild scientific hunch.
Thank goodness.
They had been loud—gasps, moans, the rustle of bodies—and both Gen and Senku made a mental note to watch their mouths next time. The last thing they needed was their friend bursting in on their 'research.'
***
The next time happened later that night.
As a mentalist, Gen knew the tension between her and Senku hung thick in the air after what had transpired that morning. She could sense it like a taut wire, ready to vibrate at the slightest touch. So she anticipated that once they were alone, that tension would snap like a bowstring.
Gen had steered clear of Senku all day, and in return, he hadn't sought her out.
She'd prepared herself with a herbal drink—a rudimentary birth control concoction from the Ishigami Village, possibly dating back to Byakuya Ishigami's era, and refined by Senku's ingenuity, from what she'd overheard around the village.
When night finally fell and everyone was surely asleep, Gen made her move.
Senku arrived after his bath in the nearby lake—Gen knew his routine well.
After he arrived, they lay side by side for a while, the silence heavy, until she was certain Chrome had drifted off downstairs.
The air between them crackled with unspoken desire, their breaths syncing in the dim hut, every shift of the futon amplifying the electric pull drawing them closer. Gen's heart pounded, her skin prickling as if charged, while Senku's steady gaze in the low light betrayed his own restrained hunger.
When Gen knew for sure that everyone in this village was dead asleep…
… That’s when the tension snapped.
Gen turned and straddled Senku, towering over him this time, and captured his lips in a fierce kiss. Senku responded without hesitation, his mouth meeting hers with equal fervor, hands rising to cradle her face.
Their kisses deepened as they shed their clothes, lips parting only to tug at fabrics. Senku's fingers worked the ties of Gen's top, peeling it away to expose her breasts, while she yanked his clothes out, and their mouths crashing back together in between.
Tongues tangled hot and urgent, breaths mingling as Gen's hands roamed his chest, nails grazing his skin.
Senku's palms slid down her sides, his lips trailing to her jaw, then neck, sucking lightly as she kicked off the last of her undergarments.
There’s one thought in her mind as Senku kept kissing her…
The fact he became such a good kisser.
All the stolen kisses from that week had honed his skill. Senku kissed with a newfound expertise now, his tongue exploring her mouth confidently, drawing soft whimpers from her as they finally lay bare against each other.
Once fully naked, Gen wrapped her hand around Senku's thick cock, feeling its heat and girth pulse in her grip.
Senku inhaled sharply, his hips bucking slightly. She pumped him slowly, a few deliberate strokes to feel him harden further under her touch, but nervousness fluttered in her chest like trapped birds.
Her free hand trembled as she positioned herself above him, thighs quivering, a knot of anxiety twisting in her gut—this was it, the real step, and doubt whispered that it might overwhelm her.
She couldn’t help but glare at Senku’s dick. Because…
Of course he's so damn overconfident about everything, Gen thought with a wry internal chuckle, with a cock like this. It's basically a scientific marvel with Senku-chan’s long, thick, and probably engineered for maximum ego boost. No wonder he acts like he can conquer the Stone World single-handedly.
“Wait!” Senku's hands flew to Gen's hips, steadying her. “We should prep—”
“I already did.” Gen smiled, though a deep blush colored her cheeks. “While you were out at the lake.”
“Oh.” Senku flushed too, then shot her a smirk. “Impatient, are we?”
She rolled her eyes. “As if you ave-hay a leg to stand on there.”
After that, Gen guided the head of Senku's cock to her entrance, slick from anticipation but still tight with inexperience. Senku helped, his fingers pressing gently into her hips as she began to lower herself.
The stretch came immediate and sharp as the burning ache of his thickness pushed past her folds, her walls resisting the intrusion.
It hurt more than she'd imagined, a deep, stinging pressure that made her twist her face in discomfort, breath hitching as she paused halfway down.
She'd chosen this position deliberately, straddling him to ride at her own pace, because she'd heard it gave women control during their first time—allowing them to ease onto the penetration, stop when needed, and avoid the deeper thrust of being beneath. It let her dictate the depth, turning vulnerability into agency amid the raw sensation.
Senku's brow furrowed with worry. “Hey—”
“I'll be ine-fay.” Gen panted, halting for a moment to let the pain ebb. “I just need to get used to this.”
“That's fine and all, but Gen…” Senku gripped her hips tighter, his voice low and earnest. “If you want to back out, just say so—I won't complain. If you need it slower, I'll help you through it. Remember: this is your body, we go at your speed.”
Gen couldn’t help but smile at Senku’s reassurance.
His words touched her deeply, melting some of the tension in her chest and sparking determination.
With that reassurance, she steeled herself and sank lower, inch by inch, until she felt the full length of Senku's cock buried inside her, filling her completely.
She noticed Senku's fists clenching the fresh primitive bedding they'd changed that morning, knuckles white. Unintentionally, her inner muscles clenched around him in response, drawing a loud groan from his throat.
Gen slapped a hand over Senku's mouth in panic.
“Senku-chan,” she reprimanded in a hushed whisper, “Chrome-chan's right downstairs—you'll wake him up.”
Senku grimaced but nodded beneath her palm, eyes dark with restraint.
Then, Gen began to adjust, rocking her hips tentatively at first—small, shallow circles to let her body acclimate to the fullness, the initial burn fading into a fuller, throbbing ache as her wetness eased the way.
A second later, she lifted slightly and lowered again, more smoothly this time, testing the rhythm.
Senku moaned against her hand, his face contorting in pleasure.
Before Gen could lift and lower herself again, a faint murmur escaped Senku's lips beneath her hand.
At first, Gen did not understand what Senku was saying. But as the moments passed by, that changed.
"Scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium,” Senku kept mumbling as his voice a low with rhythmic chant, eyes squeezed shut as if reciting a lifeline. “germanium, arsenic, selenium, bromine, krypton, rubidium, strontium, yttrium, zirconium…”
Gen froze, staring down at him in disbelief. "Are you..." She pulled her hand away slightly. "Are you mumbling the periodic table of elements right now?"
Senku's eyes snapped open, a flush creeping up his neck. He looked mortified, caught in the act.
"What if I am…?"
"I'm offended," Gen said, her tone playful but pointed. "A beautiful lady like me is ight-ray in front of you—riding you—just so you know, Senku-chan."
"Exactly," Senku shot back, his voice strained with a mix of defensiveness and desire. "Why do I need to keep mumbling the periodic table of elements when you're right here in front of me?"
For a moment, Gen blinked at him, confusion flickering across her face.
She'd been so wrapped up in her own sensations—the ache, the stretch, the tentative rhythm—that she hadn't fully registered how deeply this was unraveling Senku.
His cock throbbed inside her, hot and insistent, and the way his chest heaved told her everything.
Then she grinned, a wicked spark lighting her eyes. "Were you about to come, Senku-chan?"
Senku fell silent, his jaw tightening, but the deepening red on his cheeks gave him away.
Gen's grin widened. She loved seeing him like this, so unraveled especially how unflappable the scientist was reduced to stammers by her body.
Eager to push him further into that vulnerability, she clamped her hand back over his mouth and sank down harder, taking him deeper in one deliberate motion.
Senku's muffled moan vibrated against her palm, his hips jerking up involuntarily.
Gen bit her lower lip hard to stifle her own gasp, the sharp twinge of pain flaring where their bodies joined—her walls still tender from the initial breach, stretched taut around his girth.
She started riding him then, lifting her hips in a slow, experimental rhythm, her thighs burning with the effort as she rose and fell.
Each descent sent a jolt through her: the burn of friction where she was still raw, mingling with a building heat that spread from her core, making her slit throb against the base of his shaft.
It hurt, that persistent sting like overworked muscles protesting, but…
The pleasure crept in relentlessly too—waves of fullness that made her toes curl, her breath come in shallow pants.
To muffle the sounds threatening to escape, Gen leaned down and crushed her lips to his, swallowing his groans as their mouths fused in a messy, desperate kiss. Senku responded fiercely, his free hand tangling in her hair while the other dug into her hip, guiding her pace.
He thrust up to meet her, shallow bucks that drove him deeper, their tongues sliding slick and urgent, breaths ragged between seals of lips and teeth nipping softly.
The dual sensations overwhelmed her: a deep ache that made her wince a little even as the pleasure swelled, coiling tight in her belly like a spring ready to snap.
Her cunt clenched around him involuntarily, slickness easing the way but not erasing the rawness, turning every rock of her hips into a push-pull of discomfort and ecstasy that left her trembling.
Senku came first, his body tensing beneath her as he broke the kiss with a choked grunt, spilling hot inside her in pulsing jets. The feel of it triggered her own release moments later—a shuddering climax that ripped through her, walls fluttering around his softening cock as stars burst behind her eyelids.
Gen pulled herself off him with a grimace, the drag of his length from her sensitive folds sending a final sting through her core. Senku winced too, but the pure bliss softening his features—eyes half-lidded, lips parted in a dazed smile—made it all worth it.
She then lay next to Senku, her body still humming from the aftershocks, sweat cooling on her skin as she caught her breath.
"So?" Gen asked, propping herself up on one elbow to look at him. "How's this experiment going?"
Senku stared at the ceiling, his chest rising and falling in a lazy rhythm, a dopey grin tugging at his lips despite the scientific fog in his brain.
"Brilliant. Absolutely brilliant," Senku mumbled, his voice thick with post-orgasm haze. "The endorphin release... dopamine flooding the synapses... it's like a chemical fireworks show in my prefrontal cortex, but ten billion percent more... euphoric."
The words tumbled out in his usual analytical drone, but they clashed hilariously with the way his eyes glazed over, like a professor lecturing through a blissed-out stupor.
Despite herself, Gen laughed while also agreeing with Senku.
Sex really was brilliant, it’s messy but utterly going to be addictive. She could still feel the ache between her legs, a reminder of how he'd stretched her wide.
"This needs further experimentation," Senku declared eventually, his tone snapping back to that trademark seriousness, though his hand idly traced circles on her thigh.
She shook her head in exasperation, already anticipating this. "Yeah, yeah."
"No, I'm serious," he said, turning his head to meet her gaze, his ruby eyes sharp despite the flush on his cheeks. "I know there's some positions we haven't tried yet like posterior entry for deeper penetration or lateral coital alignment to maximize clitoral stimulation. And some stimuli you might respond to, or I might, on our bed... er, any kinds of surfaces. Variables like friction coefficients, pressure points, rhythmic oscillations..."
Gen fixed him with a deadpan stare. "Kinks and positions. That's what you're alking-tay about?"
"Yes. That," Senku confirmed without missing a beat, unashamed as ever. "I also know you're flexible, with your mentalist contortions and all. We could test flexibility thresholds in ways that optimize leverage during thrusting."
"That's beside the point," she shot back, though a smirk played on her lips. "I hought-tay this arrangement was going to be purely for stress relief after the end of our first time sex? Well... it feels like that's what it's heading toward."
"And you were correct, mentalist," Senku chuckled, the sound low and satisfied, like he'd just cracked a tough equation. "But we're also conducting experiments. Scientific progress demands it."
"Right," Gen sighed, rolling her eyes.
This guy was so shameless, turning their sweaty tangle into a research project without a hint of awkwardness.
"Don't worry, Gen," he added, his voice dropping to that confident drawl. "I already have a mental model mapped out on how to pleasure your body next time—targeting erogenous zones, monitoring physiological responses, calibrating thrust angles for optimal G-spot activation."
Gen snapped her head toward him in disbelief, her cheeks heating up again. Senku just looked proud, like he'd invented fire all over.
"Care to elaborate on that, Senku-chan?" she asked, her voice laced with nervous curiosity.
"Body mapping, autonomic reactions, sweet spots," he continued, gesturing vaguely as if diagramming on an invisible chalkboard. "Neural pathways lighting up from nipple stimulation to cervical pressure... and..." Senku sat up abruptly, his gaze drifting downward. "Another development for your cunt."
Before Gen could even process the blunt word—let alone ask what he meant—Senku turned them around and crawled between her thighs, his fingers brushing her slick, swollen folds without warning.
She couldn't help but flinch, the overstimulation hitting like a spark on raw nerves, her hole still tender and pulsing from their earlier fuck.
"Um, Senku-chan—"
He hummed thoughtfully, ignoring her protest as he lifted her hips with gentle but firm hands, tilting her pelvis up for a better view.
Gen's mortification burned hot.
There she was, legs splayed, her freshly fucked cunt on full display under his intense scrutiny, lips puffy and glistening with their mixed cum.
Senku inspected her like a prized specimen under a microscope, his face inches away, eyes narrowed in fascination.
"See this?" he murmured, tracing a light circle around her entrance with his fingertip, making her jolt. "Fingering isn't just about insertion—it's technique. Curling the digits to hit the anterior wall, varying pressure like a piston mechanism, building lubrication through rhythmic strokes. The same principle applies to the dick: not brute force, but precise angles, girth utilization for stretch response, and tempo modulation to sync with your contractions. It's all about engineering maximum friction with minimal discomfort."
Gen yanked herself away from his grasp, scrambling back a bit as she glared at him fiercely, her face flaming.
"Hey, you're so shameless! Don't do that to a lady—it's embarrassing-way to have you staring at my vagina like it's some lab rat!" She hissed at him.
If Chrome hadn't been snoring downstairs, she would've shouted it loud enough to wake the whole village.
Senku blinked up at her, genuinely confused, his brow furrowing. "It's not embarrassing, though? We just had sex. Your cunt's right there, post-coital and all—data collection is the logical next step."
Gen smacked him lightly on the head, the thwack echoing softly in the dim hut.
This guy has no shame!
Treating my most intimate parts like a petri dish while I lay there feeling exposed and oddly amused…!
And that was the start of Senku and Gen experimenting with sex—positions, touches, and techniques that pushed boundaries with Senku's relentless curiosity and Gen’s playful pushback—eventually turning them into masters of each other's bodies, blending science and sensation in ways that left them both utterly spent and satisfied.
Gen found out, in the end, that they were both freaks when it came to sex.