Work Text:
“So when is your birthday?” Senku asked Gen.
It had only been yesterday that Senku had been given an observatory hut, and that had been Gen's birthday gift to him. Sure, the villagers had provided the manpower, but none of it would have happened if Gen hadn't taken the initiative to give him a present and instructed Kaseki on how to build an observatory.
There’s also a fact Gen counted the date of Senku’s birthday.
Before that, Senku hadn't trusted Gen. He had known how shallow she could appear, and he had been convinced there was always a chance she would turn her back on him the moment the odds of losing grew too high.
But Senku had been completely wrong in his judgment.
The realization left him with the tiniest hint of guilt for ever doubting her.
Honestly, he could almost hear Byakuya's voice scolding him for judging people too quickly, followed immediately by another lecture about how he should've taught Senku better. The thought nearly made him roll his eyes. Stupid old man.
Senku was usually a good judge of character. After all, he had figured out how suspicious Tsukasa had become regarding Senku’s plan to save all of humanity.
But Senku wasn't perfect.
And one of his biggest misjudgments had been Gen.
Now, Gen stared at him from her bed with her head tilted slightly, already getting ready to sleep.
“Why are you asking?” she asked warily.
“So I could return the favor.” Senku shrugged. “When your birthday comes around, we can celebrate—”
“No.” Gen cut him off with a smile, but her tone had come out far sharper than the expression on her face.
Senku blinked. “…Uh.”
As though she had realized how harsh she'd sounded, Gen grew sheepish for the briefest moment. Then, just as quickly, she slipped an easy, pleasant smile back into place.
The mask was back.
That had always been one of the reasons Senku hadn't liked Asagiri Gen at first. She shifted between emotions so effortlessly that it had always unsettled him.
But now, knowing how badly he'd misjudged her before, he found himself more intrigued than wary. He was beginning to suspect that beneath all those carefully crafted expressions, Gen was much softer than she let anyone see.
He was getting used to her.
“Sorry, Senku-chan.” Gen smirked playfully. “But my birthday is ighly-hay confidential.”
So that was it. Gen didn't want him to know.
And, more importantly, she didn't want to celebrate it.
“Alright.” Senku shrugged.
He ignored the sigh that escaped her afterward. It might have been a relief. Or maybe disappointment.
Honestly, Senku couldn't tell.
Whatever Gen wanted, he decided he would respect it.
***
Numerous things had happened since they conquered the Tsukasa Empire, locked Hyoga and Homura in a cage, and most recently revived Nanami Ryusui—the captain they needed for their voyage.
Then, he found out people wanted to celebrate the day of Senku’s revival.
Senku wasn't entirely sure who had initiated the idea, but Ryusui had been absurdly delighted that Senku’s revival had fallen on April 1. Fifteen days from now.
Actually… It was probably Ryusui himself.
The man had insisted that being brought back to life deserved a proper celebration, and he obviously loved parties.
So Senku had declared the day everyone's day off.
“Senku!” Gen approached with a smirk, gesturing toward the cluster of children gathered behind her. “They're looking for you.”
Senku glanced past her. A dozen eager faces stared back at him expectantly.
He raised an eyebrow. “And what did I do this time?”
“You promised to play with us!” one of the boys protested.
“Yeah!” a little girl chimed in indignantly. “You told us yesterday!”
A chorus of agreement erupted from the rest of the group. Suika nodded enthusiastically along with them.
From the sidelines, Senku caught sight of the adults watching the scene with quiet amusement.
Before the children could swarm him, Gen stepped subtly into their path, shielding Senku from the eager crowd.
I can distract them if you want, she said with nothing more than a small smile and a slight tilt of her head.
Senku snorted. I'm fine, Mentalist.
Gen only shrugged. Softie.
She strolled away without another word. Senku rolled his eyes after her.
It wasn't that he was soft.
He had simply made a promise. And Senku Ishigami wasn't about to break one.
“Alright.” Senku clapped his hands together, drawing every child's attention. “Let's play.”
A chorus of excited cheers rang out before several small hands grabbed him and eagerly pulled him toward the open field.
This wasn't the first time Senku had done this.
Whenever work slowed down enough to give him a break, the children of Ishigami Village somehow managed to claim a portion of his time. They would pester him into joining whatever game they had in mind—or, more often than not, let him invent one instead.
Maybe he was a little fond of them.
Well, in a way… They were all pieces of Byakuya's legacy.
“Listen up.” Senku crouched down, picking up a handful of pebbles and a few sticks from the ground. “Today's mission is to build the tallest tower that can survive an earthquake.”
The children stared.
“...An earthquake?” one of them echoed.
Senku grinned. “The earthquake's me.”
That immediately earned a wave of determined protests.
Teams formed almost instantly. Some children stacked stones as carefully as Kaseki at work, while others argued over where each stick should go. Suika darted from group to group, proudly delivering ‘construction materials’ from her helmet.
Once every tower had been completed, Senku marched down the line, drumming his fingertips against the ground to imitate small tremors.
"Let's see how this one holds up..." He lightly shook the base. "One second..."
The tower immediately tipped over.
"Structural failure." Senku snorted as the children groaned.
At the next tower, Senku gave the ground another series of small shakes.
"One, two, three—" He cuts himself as the stack wobbles before collapsing.
Another group of children both groaned and pouted, and he chuckled.
Senku shook his head and patted one of them. "It’s better, but try a wider base next time."
The kids nodded their heads, and seemed to listen to him.
At the last tower, he kept shaking the ground.
"One, two, three, four, five, six, seven." It remained standing.
Senku smirked.
"Ten billion points for this one.”
The children erupted into cheers as though they had made a scientific breakthrough rather than a pile of rocks.
By the end, every fallen tower had become another lesson. Senku showed them why wider bases held better, why balancing weight mattered, and why rebuilding was always faster after learning what had failed.
Without realizing it, the children had spent the entire afternoon learning physics.
Eventually, the games came to an end.
As the children scattered, several of them spotted Gen walking back toward the where Senku and Gen share a hut.
“Gen!” a little girl called, bouncing over to her. “Can you play with us tomorrow?”
Gen paused her steps, and blinked towards them.
“Please!” Suika added, clasping her hands together.
The rest quickly joined in, voices overlapping until they all blended into one loud chorus of hopeful pleading.
The sight almost made Senku laugh.
Come to think of it, besides himself, Gen probably spent more time entertaining the village children than anyone else.
Gen placed a hand dramatically against her chest.
“Well,” she said with a thoughtful hum, “a famous mentalist does have a very busy schedule...”
The children collectively gasped.
“…But I uppose-say I can squeeze you all in.”
“Yay!”
“I promise I'll play with you tomorrow.” Gen chuckled.
To seal the promise, Gen crouched to the children's eye level and held up three smooth pebbles. With a flick of her wrist, they disappeared. Tiny hands flew to confused faces as the children searched the ground in disbelief.
Then, one by one, Gen plucked each pebble from behind a different child's ear, earning delighted squeals and wide-eyed laughter before finishing with an exaggerated bow.
The children burst into cheers, surrounding her for a brief moment before racing off to tell everyone about the incredible magic trick they'd just witnessed.
Once they had gone, Senku looked over at Gen with the corner of his mouth curling upward.
“Softie,” he muttered.
Gen rolled her eyes, though the smile tugging at her lips never disappeared.
Together, they made their way back toward where they share a hut, where most of their supplies were stored.
Well...
Technically, they still borrowed half of them from Chrome's hut.
“You know, Senku-chan...” Gen said casually, hands tucked behind her back. “You'll make a great father someday.”
Senku promptly choked on his own spit. “...Where the hell did that come from?”
“From atching-way you spend the afternoon patiently entertaining a crowd of children?” she replied with exaggerated sarcasm. “Gee, I wonder.”
“First off, I wasn't entertaining them. I was teaching them structural engineering.”
“You called it a game.”
“It was a game.”
“A game where every tower ollapsed-cay.” Gen hummed thoughtfully. “So educational.”
“Ten billion percent.”
“So, my point still stands.” She laughed.
“Please don't say things like that.” Senku rubbed the back of his neck with a grimace. “Being a father means having a family.”
“And?”
“And having a family usually involves romance.”
“Oh my!” Gen gasped dramatically, pressing both hands to her cheeks. “Our fearless leader is afraid of romance!”
“I'm afraid of wastefulness.”
“So romance is asteful-way?”
“It hijacks people's decision-making skills.”
“What a wonderfully scientific way to avoid admitting you're embarrassed.”
Senku scowled at her. “I wasn't embarrassed.”
“You choked earlier.”
“I inhaled wrong.”
“Mm-hm.”
“It was a respiratory malfunction.”
Gen covered her mouth, trying and failing not to laugh.
For a while, they simply walked in companionable silence.
Then Senku spoke. “You'd make a good mother too, you know.”
Gen stopped walking. So did Senku.
She stared at him with wide eyes. For once, there wasn't the faintest trace of a rehearsed smile on her face as light flush spread across her cheeks.
Only then did Senku realize something, because his voice hadn't carried even a hint of sarcasm.
He had meant it.
...Well.
That explained why his own face suddenly felt warmer than usual.
“That's—” He cleared his throat. “Don't get the wrong idea.”
Gen continued staring.
“I just meant...” Senku frowned, trying to reorganize his thoughts into something less awkward. “Objectively speaking, you're good with people. The kids trust you. You keep everyone working together without making it feel forced.” He scratched his cheek. “Statistically, that's a pretty solid qualification.”
There. Scientific.
Much better.
...Why did that somehow sound even more embarrassing?
He mentally cursed himself for overexplaining.
Gen suddenly laughed, making Senku abruptly glance at her.
The laugh is not one of her practiced performances, it’s a real laugh. It pulled Senku right out of his internal self-criticism.
“I'll take that as a ompliment-cay,” she said, smiling with effortless composure once more.
Then she walked ahead.
Senku let out a quiet sigh before following her.
The more he thought about it, the more convinced he became that he'd been right.
Gen really would make a wonderful mother.
Not just because she was good with children. She had a way of understanding people that almost no one else did.
Whether it was smoothing over disagreements between the former Tsukasa Empire and Ishigami Village, keeping everyone's spirits up during exhausting projects, or quietly noticing when someone needed encouragement, Gen somehow managed to hold the group together without ever asking for credit.
Maybe that was the mentalist in her. Or maybe… That was simply who Asagiri Gen was.
Senku frowned thoughtfully.
Gen deserved something good in return.
Just like the observatory had been Gen's gift to him, he wanted to give her something she would never expect.
The only problem was… Gen refused to tell anyone when her birthday was.
Senku let out a frustrated sigh.
Then an idea struck him.
… Pulling it off wouldn't be easy.
They slept in the same room, planned every project together, and worked side by side almost every waking hour.
Keeping a secret from the world's greatest mentalist was probably one of the most difficult things he could attempt.
...Which only made him want to try.
***
It was Revival Day.
Ryusui had gone completely overboard with the celebration.
Which, admittedly, was exactly what everyone had expected.
Colorful banners woven from dyed cloth fluttered in the breeze while enormous platters of roasted fish, freshly baked bread, fruit preserves, and every luxury the Kingdom of Science could currently afford covered long wooden tables. Music echoed throughout the village as someone beat drums while others clapped along to the rhythm. Even Chrome had somehow been convinced to join one of Ryusui's ridiculous contests, much to everyone's amusement.
Strangely enough… Senku barely saw Gen.
At first, he assumed she was simply making her usual rounds through the crowd, chatting with everyone as she always did.
But every time he caught sight of her dark and white hair in the distance, she disappeared again before he could reach her.
Eventually, he started asking around.
“Gen?” Chrome scratched his head. “She was here a minute ago.”
“I just talked to her,” Kohaku said. “She left right after.”
“She helped me set the tables,” Minami chimed in. “Then she went somewhere.”
Every answer was the same. Gen had been there, and then she hadn't.
It was... statistically unusual.
Normally, Gen drifted through every conversation in the village with effortless ease.
Today, though, she seemed determined to stay just out of reach.
Not that Senku had much opportunity to chase after her. The moment one conversation ended, another began.
The villagers wanted to thank him for everything he had done for them. Chrome insisted he judge another contest. Kaseki proudly showed off improvements to several tools. Ryusui dragged him into yet another celebratory toast despite the complete absence of alcohol.
Even the children refused to leave him alone for more than a few minutes before demanding another story about the old world.
It made sense.
This was the anniversary of the day Senku had awakened alone in the Stone World.
To everyone else, today celebrated the beginning of everything.
To Senku… It was simply another day.
His fingers brushed against the small box tucked safely inside his pocket. It suddenly felt much smaller than it had that morning.
By the time the festivities finally wound down, the sun had long since disappeared beyond the horizon.
Senku still hadn't found Gen.
With a quiet sigh, he made his way toward his hut.
“Long day?”
Senku immediately turned toward the familiar voice.
There sat Gen near the wall, calmly combing her long hair as if she'd been there the entire time.
“…What the hell?”
She looked up innocently. “What?”
“Where were you?” Senku asked, unable to keep the question from slipping out.
“Oh?” Gen blinked before tilting her head, a teasing smile already forming. “Did you miss me, Senku-chan?”
Senku let out a long groan. Without another word, he reached into his pocket and tossed the small box in her direction.
As if acting on pure instinct, Gen dropped her comb and caught it neatly with both hands.
Gen frowned in confusion before opening the small box.
The moment she saw what was inside, her breath caught.
“I don't know when your birthday is,” Senku said, leaning casually against the wall. “So I figured Revival Day was as good a day as any.”
He shrugged his shoulders. Besides, this day was a perfect time to give a gift to someone.
“You've probably been missing your poker cards. Just think of it as me returning the favor for what you did for me.”
Gen carefully lifted the deck from the box, turning it over in her hands as if she’s afraid she might damage it.
“You... ade-may these?” she asked quietly.
“I did.”
“And you drew them?” Her fingers traced the intricate drawings on the backs of the cards.
“Yeah.” Senku rubbed the back of his neck as he looked away. “It's no big deal.”
It really wasn't.
Compared to the observatory Gen had given him, a handmade deck of cards hardly felt equivalent.
Gen was silent for a long moment. Then she smiled but not the polished smile she wore so effortlessly.
“This...” Her voice faltered. “I love it, Senku-chan.”
Something in the way she said it caught him off guard. Just quiet sincerity.
For someone who hid behind smiles as naturally as breathing, the honesty in her voice felt almost private.
It’s like he had been allowed to glimpse something no one else was meant to see.
Gen gently set the cards back inside the box before placing it on a nearby table.
Then she crossed the room toward him, the air between them tightening with every step.
Without saying a word, she grabbed the front of his shirt and planted a hard, assertive kiss on him, her tongue already moving down the seam of his mouth rather than a gentle, hesitant touch.
For the briefest moment, Senku's mind went completely blank.
Then he returned the kiss with quiet familiarity, resting a hand lightly against her cheek but his other hand found her hip, fingers curling into the fabric of her clothes, dragging her flush against him. She gasped softly into his mouth, and he felt the tremor run through her.
When they finally drew apart, Gen stayed close enough that their foreheads nearly touched, her breath coming in short, warm pants against his lips. Her fingers still twisted in his shirt, knuckles brushing the bare skin just above his collarbone.
“Someday,” she whispered, “I promise I'll tell you when my birthday is.”
“Then get excited, mentalist.” A small smile tugged at the corner of Senku's mouth. “We'll celebrate it.”
“…Yeah.” Gen's expression softened, but there was joy in her eyes too.
There was the faintest hesitation in her answer. Her thumb traced a slow, deliberate path across his chest, stopping just shy of his heart.
Before Senku could ask about the hesitation, she closed the distance between them once more, silencing the question before it could leave his mouth.
This kiss was more deliberate, slower, and deeper than the others. Gen pulled him down into her as her hand moved from his shirt to the back of his neck, her fingers digging into his hair. His fingers tightened on her hip, pressing her till there was no room between them, and he let out a low moan.
Senku let out a quiet, amused breath against her lips, his thumb brushing the curve of her waist, dipping just beneath the hem of her shirt.
Maybe… Gen really liked the gift.
And he intended to show her exactly how much he appreciated that.