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Ishigami Byakuya had never expected or really planned to have children, but when Senku came into his life he took one look at those curious red eyes and decided that he was going to be the best dad ever no matter what.
Senku, like with nearly everything he did, didn’t make it easy for him.
When people described their children as tornadoes, Byakuya assumed they meant finding them coloring on the walls or leaving legos in the path of bare feet, not taking the toaster apart at six am to figure out how it worked or burning holes in the kitchen counters with citric acid experiments. Senku, behind his adorable face, was far too brilliant for his years, painfully blunt, and about as emotionally intelligent as a rock, and Byakuya could not love him more if he tried.
It had taken a fair bit of trial and error, and Byakuya quickly learned that Senku made any and all parenting guides completely useless by being unlike any child he’d ever met, but the two eventually came to understand one another. Both had difficulties relating to their peers - Byakuya having never quite lost his childish whimsy and Senku seemingly having been born without one, so it wasn’t hard for him to notice that Senku struggled to make friends.
Senku acted like it didn’t bother him, turned his focus towards scientific pursuits, but Byakuya could clearly see the hurt of each rejection no matter how much Senku tried to hide it. Byakuya was sure many parents would recommend ways to change their child’s behavior to fit in better, point out the sharp edges of their personalities and smooth them out to be more palatable, encourage them to try harder to fit in - but from experience he knew better.
Senku didn’t need to change to suit the world, what he needed was for Byakuya to just dive in with boundless support and understanding. For every struggle Senku went through, Byakuya had decided to be there as his greatest cheerleader. As far as he was concerned, Senku was perfect just the way he was and Byakuya wanted nothing but the best for him. Forget a car, he would sell a kidney if it meant seeing the look on Senku’s face when he saw that telescope again.
Though as Senku got older, Byakuya did begin to worry that the struggles his son had encountered with making friends would only be harder when it came to romance. Byakuya loved Taiju and Yuzuriha like his own children, glad Senku had such wonderful friends, but like any father he was worried about his son ending up alone.
Of course, Senku scoffed at the notion. He poked fun and expressed his annoyance with Yuzuriha and Taiju’s blossoming romance and how “cringe” it was, rolled his eyes and made faces at romantic scenes in movies and on television, and insisted he had no interest in “such ridiculous things”.
Which was what made Asagiri Gen’s sudden appearance in Senku’s life such a surprise.
He showed up one day out of nowhere - as though Byakuya’s wish for his son to find a partner had materialized into a teenage boy on his doorstep. He wasn’t anything like Byakuya would have pictured, completely different from Taiju and Yuzuriha and not at all what he’d ever expect as one of Senku’s friends, the exact opposite of Senku in almost every way.
And yet it was clear from the get go that Gen liked Senku. He would drop by the apartment with snacks only to hang out for hours after, pester him seemingly relentlessly via text and phone regardless of whether or not Senku replied right away, poke and prod and wheedle at him until he took a break.
But even more surprising was the fact that Senku let him.
He let Gen pull him away from coding and experiments to go to karaoke with friends, listened when he lectured him about staying up too late and drinking too many energy drinks. He didn’t snap or bristle when Gen got too close or tugged at his arm. He didn’t complain - much - when Gen showed up and just sat in his room or in his lab to watch him work - if anything he complained more when the other boy wasn’t around practicing card tricks in the background while chattering about whatever random topic had caught his fancy that day.
What Senku did do when Gen was around was light up.
He talked more, laughed more, didn’t hesitate to make jokes or talk about his interests. He ate when Gen shoved food in his hands, slept when Gen told him he should, and, most importantly to Byakuya, never once tried to change himself around Gen.
Sure, Gen got him to take better care of himself on a base level, but there was never any hint that he wanted Senku to fundamentally change himself. Not how he dressed or spoke or did his hair, not his interests or favorite foods - Gen seemed to like Senku exactly as he was, rambling tangents, sarcastic barbs, frayed hoodies and mad scientist hair and all.
And when Senku did try something new, Gen was there to support it. He’d compliment a new look, tease him for there being cola in the fridge, ramble right back at him when Senku asked about a magic trick or some random thing from social media.
Acts of appreciation, not expectation.
Gen didn’t seem to just like Senku, he understood him. He was able to get Senku to eat and drink and sleep without ever making him feel nagged at or controlled. He always seemed to show up right when Senku needed a sounding board, listening to him raptly but never shying away from telling him he was wrong when he needed to hear it.
He made sure Senku had fun, the way a teenager should. Made him happy.
Byakuya had once lain awake at night, years ago, wondering if anyone would ever look at his son the way Gen did and now here he was and he couldn’t be happier.
So when Yuzuriha told him the two weren’t dating, he was baffled. It was obvious how Gen felt about Senku, and as far as Byakuya was concerned just as obvious how Senku felt in return.
There was no other explanation for the way Senku’s shoulders eased when Gen showed up. How his voice softened - just barely - when he said Mentalist like a pet name. The fact that Senku had willingly sat through multiple episodes of Jersey Shore just to understand a reference Gen was making.
Byakuya was an astronaut - he knew gravity when he saw it.
But, as with all things, it was Senku that made things difficult. For as brilliant as his son was, Senku was dangerously oblivious, frequently obtuse, and universally obstinate.
“Gen’s pretty great, isn’t he?” Byakuya asked off-handedly one evening over dinner, hoping to get some sort of confirmation that he wasn’t just imagining things.
Senku just shrugged. “He’s okay. Less annoying than most people.”
Blinking for a moment, Byakuya barked out a laugh of surprise. While most would have heard a brush off, he knew Senku better. “That might be the nicest thing you’ve ever said about someone.”
Rolling his eyes, Senku got up to bring his plate to the sink, “Don’t be weird about it, old man.”
Byakuya couldn’t not be weird about it. Less annoying than most people was practically a love confession coming from Senku - but as much as he wanted to tease and poke his son, he held back. He could tell that while Senku hadn’t quite realized his feelings that drawing too much attention to them would do more harm than good - Senku didn’t react well to pushing.
With Senku, it was better to… gently nudge.
So Byakuya did what any loving father would and meddled.
He began inviting Gen to stay for dinner more often, every so often “forgetting” an ingredient and sending the two out on a shopping excursion for him. What should take fifteen minutes often took far longer as the two lost track of time on their walks together, coming back laughing or playfully bickering in a way that had Byakuya beaming.
He would casually - always casually - comment on how bright the apartment felt when Gen was there, how pleasant and thoughtful and charming he was. How nice it was that Gen was worried about Senku and always made sure he took care of himself, how sweet it was that he never came empty handed, and how impressive that he could keep up with Senku’s scientific ramblings.
But like the cement block he was, Senku waved it off. To him, Gen treated him no differently than anyone else - he was just that kind of guy and they were just good friends.
And while Byakuya certainly loved having Gen around, after several weeks of seemingly no progress, Byakuya decided to give a firmer nudge.
When Gen popped in one Friday night as Byakuya was finishing up dinner, he greeted him at the door with a grin and a cheerful, “There’s my favorite son-in-law!”
Gen had frozen. Senku had dropped his screwdriver.
“...What?” Senku said flatly.
Gen laughed - high and nervous - looking between the two for some explanation. “Ha, straight to marriage? What’s next, harping for andkids-gray?”
Byakuya laughed it off as if he hadn’t dropped a small emotional nuke on the pair, “Maybe once you two are out of college, no need to rush on my account!”
The rest of the evening had passed with Senku scowling at him, ears flushed red, and Gen adapting to the teasing with the expertise of a trained improv actor, which had Senku scowling at him as well.
And while he couldn’t quite get Gen to call him anything like Dad, he did finally get him to agree to stop calling him Ishigami-san so he counted it as a win. An even bigger win was when Senku stopped snapping at him when he referred to Gen as his son-in-law, eventually answering to the title with no hesitation in less than a week.
It wasn’t much, but it was something.
His next attempt was a few weeks later, dragging the two into a seemingly impromptu game night. Senku was not-so-surprisingly competitive when it came to board games and Byakuya was pleased to find that Gen was just as much so.
He was also quite adept at cheating, which Senku took great pleasure in catching him at.
“Saw that,”
“Saw what, Senku-chan?”
Senku raised an eyebrow and held his hand out expectantly, Gen staring back with eyes wide and overly innocent, eyelashes fluttering theatrically with his hand fanned out in front of him for Senku to draw from.
When Senku didn’t back down or react, he sighed, slipping the card out from his sleeve. “Do you see what I have to put up with, Bya-chan?”
Byakuya, who had been watching the interaction with an amused grin, shrugged. “You’re getting sloppy, son-in-law, even I saw-“
“This isn’t even a good card!”
“It’s not?” Gen hummed, smirking impishly, “must’ve slipped away the ong-wray one… oh well, no takesy backsies~”
Senku glared, giving Gen a shove, “Sneaky bastard,”
Gasping, Gen held his arm as if mortally wounded, “Abuse! Bya-chan, are you going to let him hurt your favorite son-in-law like this?”
“You are not-“
The two began to playfully snipe at one another and Byakuya sat back with an amused grin as Gen shoved at Senku with his shoulder and didn’t pull away. He watched them bicker, leaning into each other without thinking, shoulders brushing and knees knocking under the table.
He saw how Senku’s scowl softened at Gen’s laugh, sharp edges smoothing into something fond - Senku would adamantly deny it, of course, but Byakuya had raised him and he knew every expression and every tell that boy had like the constellations he’d memorized long ago.
If that look was Senku being “just friends” with someone, he could only imagine how far gone he’d be once he finally figured it out.
The next nudge came almost naturally - over the past several months it seemed that Gen had caught on to what he was doing which made things much easier. While he never directly acknowledged it, it seemed Byakuya’s approval of his feelings towards Senku bolstered his confidence some despite the other boy’s stone-headed stubbornness.
“Are you excited for the Tanabata Festival tomorrow?”
Senku looked up from his computer with a quirked brow, “Tanabata Festival?”
Byakuya glanced at Gen perched on Senku’s bed and grinned, leaning against the doorjamb casually. “It’s gonna be a blast. Just think of all the food stalls, carnival games - fireworks!”
“Dad, we’re too old for that stuff,” Senku snorted, turning back to his desk with a roll of his eyes.
“Speak for yourself, Senku-chan,” Gen teased, a particularly impish smile curling his mouth. “You’re never too old for estivals-fay! I already went and got a new yukata and everything,”
Senku blinked, “Wait, what?” He shook his head in confusion, “Since when are you going?”
Gen blinked, feigning surprise, “Since I was invited? You were there, Senku-chan, trying to hide the rest of the photo albums Bya-chan was showing me. Remember the cute picture of you-“
“That wasn’t an invitation!”
“Sure it was,” he shrugged, holding up a finger. “I said it had been orever-fay since I’d been to a festival when Bya-chan talked about all the fun times you two used to have and we decided we should all go to the Tanabata Festival at the local shrine this year.” He pushed his lip out in a disappointed pout, brows knit up and voice lowering in clearly feigned resignation. “I was looking forward to going with you, but I suppose if you don’t want to go, me and Bya-chan will just have to enoy the ireworks-fay without you…”
Senku squawked. “With my dad?”
Byakuya grinned, “Don’t you worry, Senku, I’ll be a perfect gentleman. Have him home by midnight and everything.”
Brain visibly rebooting, Senku looked between the two in confusion before turning his attention fully to Gen. The other boy was still pouting and Senku hesitated for a moment before muttering, “I never said I didn’t want to go, just said we’re too old for all that stuff… which is objectively true.”
Gen hummed, “Does that mean I’ll see you tomorrow?”
Senku hesitated, “I don’t have a yukata.”
“I’m sure whatever you wear will be fine, I don’t mind being the only one wearing one.”
Byakuya watched as Senku shifted awkwardly under Gen’s expectant look, still not sure if he was more impressed by Gen’s ability to wheedle his son or more pitying of how well it worked. In either case, Senku ended up agreeing to go with an overly casual shrug and plans were made to meet up the next evening.
And if Senku spent most of the next morning and afternoon distracted, irritable, and suspiciously focused on his hair and outfit, Byakuya wasn’t going to say anything.
At the festival, Gen looked particularly handsome in his yukata, the subtle geometric pattern and gradient of plum to a pale lavender elevating the otherwise casual outfit. He’d pinned an understated kanzashi into his hair to keep his long forelock out of his face in the humid summer air, the pattern mirroring the one on his paper fan.
Gen always took care with his appearance and outfits, but it was impressive just how well traditional wear suited him - and Byakuya noticed Senku noticing. He saw the slight flush of pink rising to his cheeks as his gaze lingered for just a second too long before he caught himself and snapped his attention elsewhere with an embarrassed frown.
The soft smile on Gen’s face told Byakuya he had seen it too, and Byakuya lagged behind as the two started into the festival grounds, giving them plenty of space and privacy.
He saw just how close Gen stayed to his son - laughing, sharing food, leaning in to talk over the noise of the crowd. How widely he beamed when Senku won a small stuffed rabbit at the target practice game and handed it over to him without a word, cheeks flushed pink. The way Senku didn’t hesitate to take a bite of yakitori when Gen held it out to him, not even batting an eye when Gen nabbed a piece of takoyaki in return.
How the two drifted closer together as the night went on, hands practically brushing with each step as though pulled together by an invisible force.
He watched from a respectful distance when Gen leaned in to say something Byakuya couldn’t hear over the crowd, Senku snorting and shoulders shaking with laughter as the lights went dark around them. The first few fireworks went off, drawing gazes to the sky, but Byakuya’s attention was locked on the way they reflected in Senku’s eyes as he looked at Gen like he was the most interesting thing there.
Byakuya almost had to wipe away a tear, hands in his pockets and heart so full it made his chest ache pleasantly.
Love looked rather good on his son, he had to say.
And yet, despite how well the evening had gone - how Senku had blushed and stuttered when Gen said good night and made plans to meet up in a day or two, had been quietly contemplative on the walk home and nearly choked on his own spit when Byakuya pointed out how nice Gen’s outfit had been - nothing happened.
Weeks went by with not even a whisper of a change between how the two interacted. Nothing to imply that they’d made any progress in moving things along, no sign of any sort of development.
Byakuya knew when to raise the white flag. He’d planted enough seeds, sowed enough chaos - at that point he needed to just let Senku and Gen go at their own pace.
No matter how glacial a pace it might be. Byakuya would do what he did best - be there to support his son, even if it meant no grandkids until he was in his eighties.
Sighing, he turned his key in the lock and let himself in, about to call out his arrival when he was stopped dead in his tracks by Senku and Gen tangled together on the sofa in a way that had no other explanation than the obvious.
Gen had one hand fisted in Senku’s hoodie, the other threaded through his hair, Senku’s hand having found its way under Gen’s shirt as he kissed him back with surprising enthusiasm. His glasses were askew, face flushed, and both were seemingly oblivious to their new audience, too focused on the other to notice anything else.
After a moment of shock, Byakuya cleared his throat. “You know, your bedroom door has a lock on it for a reason.”
They sprang apart like startled cats.
“I wasn’t- we weren’t-” Senku sputtered, face turning an adorably flustered shade of red as he fumbled with his glasses.
Gen smiled sheepishly, “Ah… welcome home?”
Byakuya laughed, warm and loud and utterly delighted. A full belly laugh that had him throwing his head back, tears coming to his eyes, “Well, took you long enough! Is it finally time for us to have the talk, Senku? I’ve been practicing.”
Senku buried his face in his hands, “Just end me…”
“Oh, don’t be dramatic,” Gen cooed, slipping an arm around him anyway, smiling. “He’s been waiting for you to igure-fay it out almost as long as I have.”
“You’re not helping, Mentalist,” he grumbled, but made no move to pull away. If anything, he melted further into the embrace, letting Gen stroke a hand through his hair.
Byakuya shook his head fondly as watched them, feeling something settle peacefully in his chest.
His son wasn’t alone. He’d found someone to love who loved him just the way he was - who challenged him, supported him, and made him happy.
And that was all Byakuya had ever wanted.