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It was difficult to partake in the beach party while keeping his shoulderblade covered, but Gen was nothing if not crafty. A fashionable beach shirt not unlike his regular haori thrown over his bathsuit not only fit his style, but it gave him sleeves to hide his hands in beyond its original task of hiding the mark.
The only issue was that the linen fabric-
"Oi, Gen! What's that thing on your back?"
-was translucent when wet. Which ordinarily wouldn't have been a problem since Gen had only planned on sunbathing, but someone decided it'd be fun to toss him in the water.
So now here he was. Drenched, upset, and with his secret about to unravel.
Except... No other unpetrified person was around right now. So there was nothing stopping Gen from simply...
"Oh, that? It's just a tattoo," he explained with a sunny smile. "Those used to be common in the Modern World. People would mark their bodies with important mementos or for decoration."
"Ooooh," Chrome nodded in easy acceptance. "Like the marks you all have?"
Not at all. "Something of a similar concept," Gen hardly felt like continuing that line of questioning.
"Man, you must really like cola then."
And that would be his cue.
"If you'll excuse me," he gathered the wet fabric around himself, not that it'd do anything in its current state. "I need to dry off."
***
Soulmate marks, contrary to their name, had nothing to do with the traditional idea of soulmates. It wasn't some predetermined thing hanging over your head your entire life, forcing you into the arms of someone the nebulous idea of the universe decided to be your perfect match.
No; Instead, they only blossomed upon your skin once you decided to love someone, to give them your whole heart. It wasn't odd to see people with more than one mark, some wearing entire tapestries upon their backs.
It was, however, odd to find no mark upon your skin at all. It was pretty much unheard of to step into adulthood without as much as a puppy crush or a beloved best friend, or even an infatuation with a fictional character.
It had been proof enough to convince Gen he'd never get one at all. If he hadn't when there were billions of people to choose from, how could he possibly get one now that there was just a handful of them?
He had been caught by surprise by the sharp pain, akin to stabbing — and he'd know how that felt! — pulsating over his shoulder blade as he drank that first, delicious, perfect bottle of cola.
It was a miracle that he hadn't dropped the bottle, what with the way his hand spasmed. It would have been a shame to waste even a drop of that godly nectar.
It was only later that he had managed to find something reflective enough to serve as a mirror, someplace secure enough to disrobe and twist until he could see his back.
And the image of a slightly misshapen, sloppily-labelled, perfect bottle of cola now forever immortalized upon his skin.
Surprise didn't last long; Not when he gave it some thought.
Instead, resignation took its place. Because Senku was — as he would have put it — 10 billion percent dedicated to science. And not one millimeter interested in romantic pursuits. Gen doubted he'd ever warrant enough thought in that genius pile of leeks to cause reciprocatory emotions.
That was fine, though. Nothing had changed from Gen's original plan. He'd continue supporting the Kingdom of Science, continue being of what little use he could be for them, continue making sure Senku would keep his ambitious promise.
Everything else was just... Extra.
***
"Hey, Senku, can you bring back tattoos?"
"Hm?" Senku hadn't looked away from his experiment — he couldn't, not at its current stage — but he let the thought percolate through his mind. "It wouldn't be difficult. It's one of the oldest types of body modifications, there were plenty of indigenous tribes with various methods of accomplishing the task for even longer before Modern Era than the world had been petrified for. Frankly, I'm surprised to see you without that art." Then, after a second- "Why do you ask?"
Chrome had, after all, asked specifically for tattoos. Where would he have heard that word-?
"Oh, Gen told me about them!" Chrome now came close enough to peer at the experiment. "He has one that looks kinda like a cola bottle. What are you-"
Senku's head snapped up sharply, looking at Chrome with wide eyes.
"He has what?!"
"A tattoo of a cola bottle," Chrome repeated, clueless at the implications. "It was hard to see over the fabric, but I think it looked identical to the one you made. Do all tattoos look exactly like the thing they're copying?"
"Not necessarily," Senku still answered, even as his head was swimming with the implications.
Chrome wouldn't understand his reaction; Sometime in the past 37 centuries soulmate marks ceased to be a thing. But he could certainly notice Senku's distraction, especially as-
"Is it supposed to smoke like th-AAAH!"
Ah, well. Not the first time something exploded in the lab.
***
Senku couldn't understand soulmate marks. The fact that no one else in the entire world understood them, either, was a paltry consolation.
The system made no sense whatsoever. Spontaneous tattoos? Alright, believable under certain conditions. Spontaneous tattoos appearing upon strong feelings of love? That was stretching it already, but Senku could have accepted that. What was love if not the right balance of hormones and certain measurable bodily reactions?
But spontaneous tattoos forming upon strong feelings of love and taking on the exact likeness of some object in full colour? Even ignoring the connotations said object would have with the subject of the emotions? That was about as far from something Senku would accept as the bottom of Mariana Trench was from the tip of Mount Everest.
He tried not to think about soulmate marks too much, lest he get apoplexy.
It had been easy back in the Modern World, his skin marred with nothing but the nebulous concept of astrophysics and science in general, having formed so early on in his life that Senku didn't even recall what it felt like. It was just as easy when he was petrified, too busy with the plans and the counting even beside the fact that he couldn’t feel the mark for his friendship with Taiju and Yuzuriha forming. It was even simpler in Stone World, when there were so many more important things to do.
It was hard to ignore when he could feel the mark appear on his skin, staring between Gen's smug expression and the observatory he had plotted to build for Senku.
It was... inconvenient. There were so many more pressing issues, so many things to reinvent, so many steps to take before the world would be saved.
Why should he use his limited time on trying to approach the subject, when he was certain Gen didn't feel the same? The Mentalist was clearly allied with them in order to recreate their old world — it was the one constant Senku could count on when considering Gen's flighty nature and elegant turn-coat pirouettes — and, had he felt any sort of attraction to Senku, it was certain he would have made a move by now.
So Senku buried the knowledge of his latest mark — the mark that could so easily be mistaken for love of the night sky, but Senku knew better — so deep in the corner of his mind, it could almost be considered forgotten.
(Almost, not entirely. It was the same corner where he stashed the awareness of his old man's life and death post-petrification, the same one where seconds still ticked upwards with unrelenting steadiness.)
And the life went on, until it screeched to a halt.
***
By the time Senku made it to their room, Gen had already changed his clothes, drying absently at his hair.
He met Senku's eye in the slightly imperfect mirror, head tilting as he catalogued the microexpressions.
Gen turned around with a frown. "What's wr-?"
"You have a soulmate mark," Senku interrupted, stepping fully into the room and letting the doors swing shut behind him. "Chrome said- it's a bottle of cola."
Gen blanched — a testament to how open he got used to being with Senku — before collecting his tells and bundling them away.
"I do so love cola," he smiled sunnily, tone impeccable, but Senku wouldn't be fooled. He only ever smiled like that when obfuscating the truth.
"My cola, Mentalist," Senku stressed.
And yet, Gen continued to play dumb. "It truly is embarrassing, that a cola made in the Stone World surpassed that manufactured back in Modern times. Then again, how could we expect anything less from your genius-"
"Gen."
His teeth clicked as he snapped his mouth closed, barely holding on to the serenity by his nails.
"Cut the crap," Senku winced. Too harsh. "Please."
Gen blinked, letting his face fall blank. He looked... tired, like that. Older than his years.
"It won't affect anything," he promised. "I know better than to expect anything. You needn't worry, Senku."
Senku had been considering his options for the entire walk over, but it was those words that spurred him into action, launching into the one indulgent plan he had been idly orbiting since the mark first appeared.
He pulled at his buttons in a hurry, getting his coat opened just enough to bare his shoulder at Gen.
He saw the Mentalist's eyes widen at his actions, before returning to their faux-neutrality once he saw Senku's marks.
"As I said," he nodded, letting his bangs cover his face. "I know your devotion is fully to science-"
"My first — and, yes, biggest — love is science," Senku agreed, tapping at his older mark before moving on to the newer one. "But this one is yours, Gen. I'm certain of it."
He could hear the sharp inhale, the hitch of Gen's breath.
"Reciprocated love," he said, his voice shaky, filled with wonder. He gathered himself together. "But, Senku-chan: What will it change?"
Senku turned back to face him properly, pondering the question. They already spent a lot of time together, exchanged meaningful gifts- hell, they even lived together.
Senku couldn't help but laugh at the realization. "I suppose you're right. We were already acting like a couple, weren't we?"
Gen's head tilted, small — but genuine — smile dancing on his lips.
"I suppose you're right," he demurred. Not pushing. It took Senku a second to realize why.
"There are things that could be... stepped up on," he matched Gen’s playful lilt. "Taking another step, so to speak."
"Ah, Senku-chan, why leave the comfort zone? You're comfortable, aren't you?"
There it was. He grinned, approaching Gen until there was scarcely any space between them.
"Hard to define the limits of the comfort zone without some thorough experimentation, wouldn't you say?" He asked, looking up at Gen's expression carefully.
Hungry. He looked so hungry.
"I suppose," Gen licked his lips, Senku's eyes getting drawn to the movement. "I wouldn't be opposed to that."
"Good," Senku nodded, leaning even closer. Next words were breathed against Gen's lips. "No time like the present!"