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Senku didn’t like being confused. Confusion was a symptom of inefficient communication, missing data, unaccounted for variables. Confusion wasted brainpower, stalled progress, created unnecessary work.
In short, confusion irritated the shit out of him.
Which was why the current situation - the persistent, irascible mood he kept finding himself in whenever Gen was around - was so unacceptable.
Gen had always had his annoying moments - leaning too close, talking too much, poking and prodding at things he shouldn’t - but it had always been surface level distraction. Enough to notice, sometimes enough to comment on, but never enough to actually bother him.
“Senku-chan, time for dinner!”
Senku grunted, not looking away from the beaker he was carefully heating up, “Gimme a minute.”
Out of the corner of his eye, he saw Gen approach the other side of the table, peering at what he was doing but maintaining just enough distance to not interfere. Despite that, when Senku glanced up and caught how intently he was looking, the curious tilt of his head and the attentive gleam in his eyes, he felt something spike inside him.
A pricking itch under his skin, a flash of something hot shooting up the back of his neck, a jump to his pulse.
Irritation.
He snapped his attention back to the beaker, jaw tightening and eye twitching faintly. He was viscerally aware of Gen’s presence, his gaze, in a way that he usually wasn’t and it was…
Annoying.
Objectively annoying.
—
Usually when Senku found something annoying he would just limit his exposure to it. Unfortunately, it was rather difficult to limit your exposure to the person you were dating. Even more so when you cohabitated with them.
It had happened with little fanfare - one night Gen had been teasing him about something - most likely his terrible sense for romance - and Senku had just blithely replied that he should “just date me and stop complaining, then.”
Gen had blinked in surprise before smiling, making the drawn out humming sound he did when particularly pleased about something.
“Oh? Don’t ind-may if I do.”
And that had been that.
To Senku’s surprise, not much had changed. They already shared the observatory and spent most of their time in the other’s presence, so beyond their bedrolls shifting closer together and their personal bubbles overlapping just a bit more, things carried on as normal. There had been no ceremony, no cringy declarations or grand gestures, just a mutual understanding and agreement.
Sure, Gen had begun touching him more casually - a hand on his arm or the gentle press of a shoulder against his side in quiet moments - but their normal day to day remained much the same.
And because not much had changed, logically, Senku should not have changed.
Except, for some reason, he had.
Because where previously Senku would be aware of Gen in the vague sense one was aware of the clothes they were wearing, now he was aware of him. Constantly.
Aware of how Gen seemed to be everywhere.
And, worse, aware of the new tight, restless feeling that hit whenever Gen laughed or stood too close or flashed one of those aggravatingly overly pleased smiles of his when he knew something Senku didn’t.
Senku scowled down at his notes.
“Such a erious-say face, Senku-chan,” Gen cooed from where he was sprawled on the floor, playing a game of solitaire with the new deck of cards Senku had gifted him. “Has that paper offended you?”
“Yes.”
“Hmmm? How so?”
“By not solving itself.”
Gen laughed and Senku felt the feeling come back. Sharp and sudden, like a spark under his skin.
His fingers tightened around the pencil in his hand.
Annoying.
So annoying.
“…Quit that,” he found himself grumbling.
“Quit what?”
“…that.”
“Well at-thay certainly narrows it down.”
Senku looked up and immediately regretted it.
Gen was smiling at him, chin resting in his palm, expression soft and warm and amused. Senku felt something surge in him, an impulse he couldn’t quite put a finger on.
The sudden urge to go over to him, grab him by the collar and-
…and-
Senku froze.
Gen’s brow lifted slightly, eyes bright with curiosity.
The feeling spiked again.
Not irritation, Senku realized. It was far too warm for that.
Too sharp and too soft at the same time.
But if it wasn’t irritation, then what was it?
—
Normally, when Senku found himself struggling in these sorts of situations it was Gen that he would go to. He would explain the situation best he could knowing that Gen would be able to reorganize and translate it into something identifiable for him - it was one of the many things Senku appreciated about the other man.
But even as socially obtuse as Senku could be, he knew telling your partner “I keep getting annoyed and irritated around you and I can’t figure out why” would not go over particularly well.
So, instead, he did what he did best.
Observed and catalogued.
He tracked what seemed to trigger the feeling - Gen laughing, Gen smiling, Gen touching him (even lightly), Gen saying his name with slightly different inflection than the standard, Gen apparently just existing within three feet of him.
He tracked the symptoms he was feeling - increased heart rate, sudden tension in hands, mild to severe distraction, intense urge to do… something (unclear as to what).
At first, he wondered if maybe he had been right all along and he just wasn’t built for dating. Unsuited to being someone’s primary focus and all the minutiae that went along with it and that by putting a label on what he and Gen were he had introduced some sort of subconscious kill switch in his brain.
Maybe it was proof that romantic entanglements were, in fact, more trouble than they were worth.
Except the thought didn’t sit right with him.
Senku knew he liked Gen. Enjoyed his company and his sense of humor, liked talking with him and having him around - even when he was being bratty and obnoxious. Senku might not have ever had any romantic interest in others but he knew ten billion percent that Asagiri Gen, for all his faults and flaws, was someone he wanted to keep - he just needed to figure out what was going on.
Across the clearing, Senku watched as Gen entertained a group of village children. He wasn’t doing much more than basic sleight of hand tricks, but it was clear the Mentalist was in his element.
Senku tracked the movements of his fingers, the curve of his smile, the glow of pride in his eyes at the way the children gasped and cheered.
The feeling returned.
But now that he was paying closer attention, he could tell that it definitely wasn’t irritation or annoyance. It was more like… pressure. Like something building inside him.
Like wanting.
But wanting what?
Gen laughed at something one of the kids said, bright and unrestrained, head thrown back and hand raised to try and smother it, and something in Senku’s chest tightened.
And suddenly… it clicked.
“Well that’s embarrassing,” he murmured, the realization obvious in hindsight.
Because of course it wasn’t irritation.
It was the exact opposite.
—
Gen found him later, hunched over his workbench in the lab.
“Senku-chan,” he drawled, sliding into the space beside him as if he belonged there, “you ready to tell me what’s been othering-bay you the last few days?”
Senku laughed softly - of course Gen had noticed. “Nah, I figured it out.”
Gen hummed, “Oh? Care to share anyway?”
“Sure.”
And before Gen could react, Senku grabbed him by the collar of his haori and pulled him in.
It wasn’t graceful, Senku managing to catch the corner of his mouth and pressing just a bit too firmly, but a few small adjustments had them kissing more successfully.
“Yup, that was it,” Senku said when he finally pulled back.
Gen blinked at him, absently running his tongue over his lower lip, “What was what?”
There was a brief pause and Gen saw exactly when Senku realized he was staring at his mouth, eyes jumping up to his. “This. This was the annoying thing.”
“…did you just kiss me and call it annoying?”
“What? No,” Senku scoffed. “It was annoying until I figured it out.”
“…Senku-chan, darling, I’m going to need a better explanation than that.”
“That, exactly. The name.” Senku insisted, as if it were obvious. “Whenever you do something like laugh or whatever, I get this… urge. I thought it was irritation but it’s not.”
“…okay…? What is it, then?”
“This,” Senku replied, kissing him again. Learning from experience, it started out much better, softer and longer and more confident and Gen melted into it.
Senku felt one of Gen’s hands find his wrist, fingers still fisted in his collar, the other stroking up the front of his shoulder to curl against the back of his neck. The touch was firm enough not to tickle but gentle enough to still be considered a caress, perfectly adapted to him in a way Senku had never really considered but Gen always seemed to figure out.
The pressure from before surged again, but that time it had somewhere to go. Senku’s chest felt tight, his head buzzing like it was running through too many calculations at once and discarding them all in favor of what he was doing.
Gen’s mouth was warm, his lips were soft, the sound he made when Senku adjusted the angle was sweet and indulgent and did things to him.
Pulling back just enough to breathe, Senku nodded. “Yeah, that.”
“…okay,” Gen said faintly, voice quiet and pleased in a way Senku couldn’t remember hearing before but that stoked that feeling in his chest like a bonfire.
“Tch, there it is again,” he muttered, the pressure spiking at the way Gen was looking at him - eyes bright, smile warm and content.
“There it- oh!” Gen’s expression morphed with delight at the realization, “I see…”
The curve of his lips took on a devious, mischievous edge and it was only the fact that he continued talking that kept Senku from leaning back in.
“Are you telling me that whenever I so much as smile at you, you feel a- how did you put it? - an uncontrollable urge to kiss me?”
“Never said uncontrollable.”
“And,” Gen ignored his comment, “you thought that was you being annoyed with me?”
“In my defense, you’re usually being pretty annoying.”
Gen let out a laugh at that, the sound lilting and fond in a way that only made the problem worse.
Because there it was again. The pressure. The want.
Senku’s eye twitched. “...quit that.”
“Hmmm?” Gen lifted his other hand to fully loop his arms around Senku’s neck, smile widening a fraction in a clear sign of just how aware he was of what he was doing, leaning forward to rub the tip of his nose against Senku’s gently. “Quit what?”
“That,” Senku replied, already closing the distance.
Gen met him easily, the kiss lingering and deepening as Senku’s hands found his hips. His grip was firm, grounding almost, and Senku bit back a sigh as the pressure eased.
“Huh,” he muttered against Gen’s lips. “Kissing you definitely fixes it.”
“Oh? Are you saying my poor, brilliant scientist is suffering from some terrible malady that can only be healed by kissing me? How simply agic-tray.”
Senku blinked. “Yours?”
Gen’s smile turned soft again - less teasing, more quiet and certain, fingers threading into the hair at Senku’s nape. “Mm. Mine.”
The feeling surged, stronger than before. An immediate zero-to-one hundred that had his pulse spiking and fingers twitching against Gen’s hips.
Senku clicked his tongue in irritation - actual irritation at the sheer predictability of the reaction - and dragged him back in.
The kiss was different, less experimental and more possessive. Gen made a small, surprise sound, and Senku took advantage of the moment to slip his tongue past his lips. It changed everything, especially the way Gen leaned closer to him, the fingers threaded in his hair almost pulling.
Better. Definitely better.
The fingers in Senku’s hair shifted, one dropping to his shoulder while the other scratched up the back of his head in a way that made the now-familiar pressure in Senku crest and spill over into something steadier. Not gone, exactly, but… satisfied.
For approximately three-point-two seconds until the idea of kissing other places occurred to him, the pressure roaring back at the sound Gen made when Senku’s lips found the hinge of his jaw.
The hand on Gen’s hip tightened reflexively - possessively - and Senku quickly realized it wasn’t just an internal reaction being provoked, giving into the urge to bite down gently. Gen’s breath hitched, the fingers in Senku’s hair curling with sudden intent instead of idle curiosity, and Senku lost whatever remaining patience he had for anything that wasn’t this.
Gen barely noticed when his thighs met the edge of the workbench, effectively boxed in as Senku pressed in after him. A flask wobbled dangerously before Senku reached out on instinct and steadied it without breaking contact anywhere else.
Efficiency, some distant part of his brain supplied. Multitasking.
The rest of his brain was otherwise engaged.
“Senku-chan,” Gen murmured, voice soft even as he tilted his head against Senku’s onslaught, “you’re being awfully forward-fay…”
“You started it,” Senku grumbled, contemplating what the easiest method to move Gen’s shirt collar out of the way would be.
Gen laughed, “By existing in your vicinity? I can promise you that if I knew this was on the table I’d have been kissing you for weeks…”
“Tch, and you didn’t say anything?” Senku huffed, sounding genuinely offended as he gave up on Gen’s collar, pivoting his attention to the other side of his neck instead.
Gen shifted dutifully, humming his approval as his head rolled to the side, “Didn’t think you were interested in this sort of thing, honestly.”
Senku frowned, more out of reflex than any actual displeasure, pulling back far enough to look at him curiously. “That’s not- when did I ever say I wasn’t interested? I asked you out.”
Gen raised an unimpressed eyebrow. “Let’s see… dodging any physical contact beyond a high five the way Ginro-chan dodges work, distracting from or outright ignoring any flirting directed your way, and the way your face always gets when you see any sort of PDA were rather strong hints.”
“Yeah, well… this is different.”
“Hmm? How so?”
“It’s you.”
Gen was the one to close the distance that time, not that Senku was complaining. Gen kissed him like he’d been waiting for permission and now that he had it he had no intention in wasting the opportunity. It was slower than Senku’s earlier attempts but no less intense.
His arms looped more securely around Senku’s neck, and with a subtle but intentional shift he was suddenly sat on the edge of the workbench with Senku bracketed between his knees.
And when he pulled back to breathe, Senku chased after, earning a small laugh.
“You know,” he murmured, fingers playing idly with the ends of Senku’s hair, “there’s a term for this.”
“I’m sure there is,” Senku replied, nuzzling his way back into the crook of Gen’s neck. “Even more for when I get your clothes off.”
Gen laughed, “Hmm, you’ll think it’s ridiculous.”
Senku, busy with finally working on the laces of Gen’s top, absently replied. “Uh huh…”
“It’s called cuteness aggression.”
“Bullshit.” He bit down on the newly revealed skin, reveling in the gasp and shudder it earned him.
Gen laughed again, the sound breathy and far less composed than before, fingers tightening briefly in Senku’s hair. “I’m erious-say,” he insisted, voice wavering at the edges when Senku immediately did it again, teeth grazing just hard enough to make a point. “It’s when you see something you like so much your brain decides the appropriate response is violence-ngk!”
As if to prove his point, Senku bit down again, higher up his neck and sucking leisurely at the flesh in his mouth. “This isn’t violence,” he muttered, shifting slightly to the right to repeat the action. “It’s efficient.”
“O-oh?” Gen managed, voice hitching at the way Senku’s hands had started to wander with increasing confidence.
“Mmhm…” a kiss was pressed to the curve of his jaw. “Considering the current response level, the best course of action,” another, at the corner of his mouth, “is repeated exposure until the reaction stabilizes.”
Gen leaned in as Senku kissed him properly, slow and deliberate.
“So your plan is to… kiss me so much you get desensitized to it?” he murmured, not letting Senku pull too far back when they paused for air.
“More or less.”
Gen huffed out another laugh that dissolved into something softer when Senku immediately chased the sound, pressing another kiss to his lips like he was trying to catch it. “Well, far be it from me to interfere with the scientific process.”
“Good. Stay still.”
Before Gen could respond with some clever remark, Senku pulled him in again, shifting against him in a way that made the whole thing spiral from “experiment” into something far less clinical.
Not that either of them minded.
The now-familiar pressure flared - Gen’s voice, the curve of his lips, the heat of him so close - and without hesitation Senku acted on it.
Again.
And again.
And again.