When a tabloid reveals that King Atem was the one who fell first, the world becomes obsessed with uncovering the truth behind their relationship.
Forced to revisit the past, Atem and Kaiba remember the days when Kaiba first began crossing dimensions to visit the afterlife—not merely for duels, but for reasons neither of them fully understood.
What begins as rivalry slowly becomes friendship, admiration, and eventually something far more dangerous. Armed with observation, theories, spreadsheets, and absolutely no romantic experience, a king and a CEO attempt to define love through logic.
Unfortunately for them, love reaches its conclusion long before the research does.
A story about first love, second chances, hand-holding experiments, disastrous scientific methodology, and the ongoing collision between affection and reason.
This is a translation of an original work on Pixiv.
Original Title: 愛と理論 17 王、恋に落ちた日
Original Author: 葉人(@Hathor₋yuki)
Personal site: https://prideshipping.sakura.ne.jp
On the living room table sat a by-now-familiar pink-covered magazine.
Printed large across the front:
Quantum Marriage Special Issue Vol. 2: An Exclusive on How It All Began!
Atem gazed at the cover, looking rather proud.
Seto, on the other hand, had quietly pressed a hand to his head.
"…You talked again."
"Only a little. They asked how we met, so I told them — just a little."
"How much is a little."
"Something like 'every time Seto came to the afterlife, I couldn't stop thinking about him.'"
"…That's more than enough."
At Seto's low voice, Atem shrugged, just slightly.
But there was no sign of remorse. If anything, he was smiling with something like embarrassment.
"They asked, didn't they? 'Which of you fell in love first?'"
"If you hadn't answered, the world would have been at peace."
Atem had a way of commanding attention even when it came to moving the world.
A few hours later, the internet and the news were alive with it.
The headlines:
[SCOOP] King Atem reveals he was "chasing" President Kaiba.
[It wasn't quantum — the King made the first move!?]
The world stirred.
The unexpected revelation: it had been Atem all along.
Fans and media across every country began verifying from every angle — who had really moved first.
That night.
The study of the Kaiba mansion.
Seto sat quietly in his chair, watching Atem across from him.
Atem was drinking herbal tea with the air of someone slightly uncomfortable.
"…Atem."
"Hm?"
"These things are meant to stay between the two people involved."
"But for me, it's the same in the afterlife and the living world. I found you, I followed you, and we finally got to walk side by side. I don't see why that should be hidden."
Seto exhaled.
It was a sigh of exasperation — and somewhere in it, something like nostalgia.
"…You haven't changed from back then. …Not that it's a bad thing…"
"Back then?"
Seto gave a small laugh and set his cup on the table.
"When you still wore your king's face at all times, and I was just going to the afterlife for a rematch."
Atem's eyes drifted somewhere far away.
The gold of his hair seemed to hold, somehow, the light of a sun long past.
"…Strange to think about. The first time you came to the afterlife — I thought, who is this person, is this modern human serious."
"I thought, who does this one think he is, with that king's manner."
Their eyes met.
And quietly, the memories of the past began to surface.
The afterlife.
An ancient palace where golden light fell into silence.
The air of the afterlife carried something like familiarity, and yet was utterly still. Light and sound both moved slowly there, and time itself ran at a gentler pace — into that world, a vivid white cut through.
It was Seto.
He stood there, having broken through the barrier between dimensions, duel disk on his arm — carrying the same sharp edge he wore in the living world.
The moment Atem saw him, something deep in his chest pulled tight.
He had left without a word that time.
If he had put it into words, it might have crumbled — so he hadn't.
And yet, he had come after him.
"…Why did you come here?"
"Obvious. To settle things with you."
A brief silence. Then Atem narrowed his eyes, just slightly, and smiled quietly.
Nostalgia, and an unbearable gladness, dissolved together in his chest.
"…Yes. Then I'll accept. Let us fight again."
Two fields unfolded across the great hall.
At the signal to begin, cards of light swept through the air, and the two souls collided.
But it was different from before.
Atem could tell.
Seto's gaze — it was not only the hostility and obsession of before. There was a different warmth to it, something like checking for something.
When the duel ended, Seto quietly switched off his duel disk and turned away.
"Atem. Is this enough for you?"
"…Yes. With this, and the living world—"
"What do you mean?"
Seto turned back. His eyes, even in the dim half-light of the afterlife, were sharp — and somewhere, gentle.
"I'll win next time. I'll come back."
He left those words behind and turned toward the living world.
Atem could only watch his back.
Words had left him. What spread through the depths of his chest was a quiet loss — and an impossible hope.
And so the days passed.
Atem found himself remembering, at unguarded moments.
The light of the living world. The heat of the duel. The look in Seto's eyes.
Telling himself he wouldn't come again — and yet, somewhere inside, waiting.
And then.
Again, a white light cut through the air of the afterlife like a blade.
"…You came back?"
"Of course. I'm not so idle that I'd leave a rival unattended."
For just an instant, Atem's eyes went wide — and then he laughed.
Surprise, and the smallest thread of relief.
Something inside Atem had unmistakably begun to move.
And so, without anyone knowing, the beginning of the two of them quietly started.
Without anyone quite noticing, the frequency of Seto's visits to the afterlife had increased.
He would appear with plausible-sounding reasons — verifying a new card theory, surveying the structure of the afterlife — and show up as though it were the most natural thing.
Each time, Atem found himself smiling without meaning to.
"You've come again. Do you have some attachment to the afterlife?"
"I simply have a subject here worth verifying."
Saying that, Seto settled into a chair as though it were entirely expected.
Whenever Atem saw him, the air in the room seemed somehow lighter.
What kind of feeling that was, Atem didn't yet fully understand himself.
In the beginning, it was nothing but duels.
But somewhere along the way, words had begun to pass between them that had nothing to do with fighting.
Ancient memories, the weight of kingship, the loneliness of being human.
In the steady calm of Seto's gaze, Atem felt something that seemed to understand him — and so he kept talking.
As the days accumulated, Seto's presence settled deeper into his heart.
In the nights, in the stillness of the afterlife temple, Seto's profile would come to him without warning.
He tried not to think of it, but his thoughts moved that way on their own.
Could this be what they call love?
The concept of "friend" had existed when he lived in the living world as Yugi. For the king of ancient Egypt, it had not.
Reverence, loyalty, love — the boundaries between them had always remained unclear, and time had ended before they could be resolved. What was happening now was the continuation of that.
For Atem, Seto had always been someone whose category he couldn't quite fix — rival, or friend, the line between them uncertain. Living on as king in a world that extended from the ancient one, that line had grown blurrier still.
Within a relationship whose boundaries had never been clear, the category of "friend" had quietly vanished.
And in that moment, in Atem's mind, the equation formed instantly: special equals love.
From then on, Atem's words carried a faint warmth that hadn't been there before.
Even while discussing theory, he would bring in the definition of love at unexpected turns.
"Love is a force that cannot be measured by reason. A resonance between soul and soul — Kaiba, do you believe in that?"
"'Resonance' is a convenient word. It's equivalent to abandoning logical explanation."
Atem deflated, just slightly — then, undeterred, came at it from another angle. That part of his nature hadn't changed.
Each time Seto returned to the living world, each time Atem watched his back disappear, something in his chest held heat.
Meanwhile, Seto too had been thinking.
Atem's shift in manner, the frequency of what he said, the movement of his eyes.
He had tried to analyze all of it rationally.
When he ran it through the AI, the closest matching behavioral pattern turned out to be the dialogue records of a "romantic advice AI."
"…Meaning: affection. A hypothesis only. But the probability is high."
Seto exhaled slowly and closed the display.
On his face, something like a faint smile had appeared.
"…Really. To be liked by the king of the afterlife — what kind of anomaly is this."
And then he began preparing to go to the afterlife again.
If they met next time, he might be able to verify it. Whether this feeling was misreading — or truth.
The duel, as always, was fierce.
The space of the afterlife trembled, and the light of cards illuminated the walls of the great hall.
But the silence that came after the conclusion was the time the two of them shared longest.
"…A good match again today."
"Naturally. I have no reason to hold back."
In the hushed great hall, the sound of Seto's footsteps carried.
As though on a whim, Seto stopped in front of Atem.
In the depths of his eyes, there was a light slightly softer than usual.
And without any warning, Seto took Atem's hand.
Warmth passed between them.
In that instant, Atem's breath stopped.
Heat rose from the core of his body, and his cheeks colored, faintly.
"…K-Kaiba?"
"I'm only confirming something."
Those words alone, and Seto said nothing further.
Atem too lost his words, and the two of them sat there, hands still joined.
Time flowed, quietly.
Neither spoke.
But neither let go.
In the silence, Atem heard something give way, deep inside him.
There are things that are understood without any words at all.
If this feeling were to be given a name—
This is love. Without question.
After Seto left, Atem sat alone, looking at his hand, and smiled.
That smile was not the smile of a king. It was soft, like that of an ordinary boy.
Meanwhile, in the living world, Seto was looking down at the same hand.
At his fingertips — a warmth, faintly remaining.
Analyzing it as he would a verification result, Seto murmured quietly:
"Affection. The probability is… ninety-five percent."
The corner of his mouth lifted, just slightly.
It was not a smile of victory.
Something warm — closer to relief.
Seto settled into his chair and looked up at the ceiling.
And slowly closed his eyes.
Whether to accept it, or not.
In the depths of his heart, something that had never before been quantifiable was beginning to take root.
To call it love yet was too soon.
But there was no longer anywhere to run.
Seto came to the afterlife again.
Atem felt the warmth rising in his chest without meaning to, and composed himself — standing straight before the throne, keeping his face calm.
The last time. Seto taking his hand.
Since that day, a question without an answer had been turning in Atem's mind.
Why had Seto taken his hand?
An experiment? A verification? Or—
But Seto's actions always had a reason. There was always intent, not mere feeling.
Thinking that, Atem watched as today's Seto walked toward him without raising his duel disk.
That alone was enough to know: today was different.
"…You haven't come to fight today?"
"The fighting is already done. More than that—"
Seto stopped mid-sentence, and looked straight at Atem.
"…Your feelings are already laid bare."
Silence fell.
Atem's eyes went wide for a moment, then darted in every direction.
"…W-what are you talking about?"
The usual kingly composure was nowhere to be found.
His shoulders trembled slightly, his fingers couldn't settle.
Seto missed none of it.
Ninety-nine percent. Revised upward.
Seto's silence pressed in.
Atem understood. He had been seen through completely.
His pulse rang in his ears.
His face grew hot.
He had always intended to say it — someday. But when the moment came, he was afraid.
Even so, he opened his mouth.
"Kaiba, the truth is, I—"
He was not allowed to finish.
Seto stepped forward, reached out, and held Atem.
Without a word — only as if confirming the warmth of his body.
Atem caught his breath for an instant, and went still.
The heartbeat of two people rang out between them.
Seto exhaled, barely, and murmured at his ear:
"…I came to answer it."
Silence came again.
But it was not the silence of uncertainty.
Even without either of them saying anything, it had already passed between them.
At last, Atem lifted his face, slowly.
In his eyes, there was no fighting flame — only a soft light.
"…I'm in your hands."
Seto only nodded.
And held him close, once more.
The wind of the afterlife moved through both their hair.
The pride of rivals, quietly, made its sound as it became something called love.
Even after they began, there was no dramatic change to the two of them in their daily lives.
Seto was always steady, unmoved, appearing in the afterlife in exactly the same way as before.
Atem too was as he always was — composed, following his curiosity wherever it led, moving as his feelings moved.
Only — occasionally, his behavior gave him away. That was the Atem of now.
"…What's wrong. Your eyes are wandering."
"No, it's nothing… It's just that when I look at your face, Kaiba, my heart seems to get rather busy."
Seto exhaled, small.
My heart gets busy. Only Atem would put it that way.
But he also knew there was no calculation in it, no ill intent.
If anything, Seto looked on that guilelessness with something like fondness — it was, after all, very Atem.
From the time he had lived in the living world, Atem had never changed.
Unafraid of others' eyes, standing with composure, direct with everyone. And without a trace of malice.
Seto understood that.
That was why he hadn't turned away.
More than that — he had wanted to stay at his side and watch that guileless strength at close range.
There was, however, one problem.
Atem had no knowledge of love as a concept.
Seto too was unaccustomed to handling actual feelings.
Two beginners, left to manage on their own.
"Kaiba — what does one do about this?"
"About what, specifically?"
"Being together like this — there's a phenomenon where my chest grows warm and I feel the need to say something."
"…That would be one of the physiological aspects of being in love."
"I see. But to define it properly, the evidence is still insufficient."
"Agreed."
They were earnest, both of them. Which meant, in short, that they were a little lacking in sentiment and romance.
After a silence, Seto rested a finger against his chin and thought for a moment.
"Then let us build it. What is love, what is affection — we construct the theory together, and define it."
Atem's eyes lit up when he heard it.
Nothing fired him up like a theoretical challenge from his rival.
All the more so when the theme was love.
"That's an excellent idea. To understand the heart, we build theory. That is our love — yours and mine."
A faint smile appeared at Seto's mouth.
Like two scientists drawing up an experimental plan.
The two of them opened a notebook and began to break down the structure of the heart.
Can love be observed?
Where is the intersection of theory and feeling?
Is love arrived at through logic the real thing?
Arguments that resembled equations accumulated, and the distance between the two of them closed, little by little.
And then — by the time they noticed, what was there was not a construction.
It was love, already complete.
"Kaiba — it seems love finished ahead of theory."
"…So it would appear."
And so, quietly, a love that had tangled love and theory together into one was born.
The day after the two of them had established their theory, Seto wasted no time.
"Now that the theory is built, the next step is verification. Physical contact in romantic relationships. We observe the effect."
"That makes sense. Where do we start?"
At that question, Seto exhaled slightly and set a stack of documents on the desk.
Atem's eyes went wide.
"Don't tell me… an itemized list?"
"Naturally. Observation is something you conduct with a plan."
"…This isn't a ritual, you know. Does observing love really need an itemized list?"
Item 1: Hold hands.
Item 2: Look into each other's eyes.
Item 3: Embrace.
Item 4: Stroke hair.
Item 5: Kiss.
And Item 6 onward: to be added as needed.
Atem held back his laughter and composed a serious expression.
"Let's not go in order — let's try things as they come to us."
"…Non-linear verification. Not bad."
And so the love experiment began.
Atem acted on curiosity alone.
Seto was to be the calm observer, keeping records.
That was how it was supposed to go.
Take the morning greeting, for instance.
"Good morning, Kaiba."
"Ah… good morning. …Why are you so close?"
"I'm checking how heart rate varies with distance."
"…The subject of observation is throwing the researcher into disarray."
Or the time Atem sat reading documents with their hands joined, and Seto's brow furrowed slightly.
"Handling paperwork one-handed is inefficient."
"But whether concentration rises or falls in this state — isn't that interesting?"
"…You're a runaway experiment dressed in a researcher's skin."
But Seto too was quickly caught in that research.
The more he observed, the more unknown phenomena appeared.
He had meant to be a calm recorder — and found he could no longer keep his own pulse from rising.
Night.
Item 5: Kiss.
After that verification was complete —
the distance between them had closed to something theory could no longer measure.
Atem asked:
"…What's the next item?"
"There are no more items on the list."
"Then let's add one. 'Spending the night together.'"
"…Unobservable territory."
"Isn't the unknown exactly where a king and a scientist should go?"
"You mad scientist."
And the night deepened, quietly.
A dizzying, sweet time passed, and morning came.
In the light, Atem was sleeping beside him.
Seto reached for the notebook on the pillow — but sat for a long moment looking at the page without moving.
The pen never touched the paper. Eventually, a rueful smile escaped him.
"…I abandon the record-keeping."
At his voice, Atem opened his eyes.
He smiled, still half-asleep.
"It seems love produces results ahead of theory."
"Every law breaks down when you're involved."
"Then make new laws. The two of us — love and theory."
Seto smiled.
This love research would certainly never end.
The more they observed, the more the heart was revised.
And today again, the two of them walked side by side, keeping records — along a path no one else had ever traveled, where theory and feeling coexisted.
The memory dissolved, all at once.
Outside the window, the night world spread out.
A quiet night after returning home.
A place the commotion of the pink-covered magazine could not reach.
A gentle time, arrived at last after a long while.
Atem tilted his wine glass and smiled, with something like nostalgia.
"Those days — we were truly feeling our way in the dark. Trying everything, looking for results… thinking back on it now, it was a dangerous experiment."
Across from him, Seto sat on the sofa with his legs crossed, exhaling briefly.
He ran a finger along the rim of his glass, and the corner of his mouth lifted slightly.
"You're still dangerous now."
Atem narrowed his eyes.
"Is that so. Why?"
"No progress… no, that's wrong. No change. The test subject who draws the observer in without a second thought — still very much present."
Atem laughed lightly and set down his glass.
Then quietly rose and walked to Seto's side, settling down beside him.
"The experiment keeps going because you don't leave."
"…I have no intention of calling it off."
"Then let's keep observing tonight as well."
Their eyes met.
Between the two of them — who should have grown — the same spark as back then ran through.
On the sofa, the red of the wine glass tipped and fell.
The night wrapped around the two of them.
Theory and records — neither was needed now.
Love is not something you observe. It is something that is, unmistakably, there.
And the quiet night passed on.
The deep, sweet night the two of them now shared.
Love and theory — still being updated, even now.
