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There were two kinds of loyalty.
The kind you gave because it was easy — and the kind you bled for because you thought it was the only thing keeping you alive.
Senku knew it the moment he saw you smile at Tsukasa after the world ended. That same smile you used to aim at him now, when you were bent over a half-built waterwheel, sleeves rolled up, sweat bleeding into the creases of your palms. Senku watched you from the riverbank, calculating the rotations-per-minute needed for optimal energy output — pretending it was just numbers clogging his brain. Pretending it wasn't you.
You, who laughed too loud sometimes. You, who argued with him until your throat was raw, stubborn and reckless and alive.
You, who had picked Tsukasa long before the world crumbled into stone and silence.
And you were still picking him. Every single day.
"Hey, genius," you called out, tossing a heavy glance over your shoulder, "you gonna help me or just stand there looking constipated?"
He huffed through his nose, dragging a hand through his hair. "Please. Like you'd survive five minutes without me, idiot."
You grinned.
It wrecked him.
It wasn't supposed to happen like this.
Senku wasn't supposed to feel things.
He liked numbers. He liked chemicals, electricity, algorithms — things that followed rules and laws and the elegant, ruthless clarity of cause and effect. He didn't like the way you messed all that up.
How you didn’t make any sense. How you stayed with Tsukasa even when your eyes lingered too long on Senku, even when your arguments with Tsukasa left hairline fractures in the air between you.
Cracks nobody talked about.
Senku kept telling himself it didn't matter.
Tsukasa was strong. You were safe.
That's all that counted, right? Logical. Efficient.
Except —
When Senku watched Tsukasa touch your arm, possessive, hesitant, like you were something breakable and already half-broken — Senku's lungs felt like they were folding in on themselves.
Except —
When he heard you at night, whispering with Tsukasa under the starlight, trying too hard to make your voice sound sure, trying not to sound like you were lying to yourself — Senku would turn over in his makeshift bed and grit his teeth until his jaw ached.
Because he knew.
He knew better than anyone what it looked like when a mind tried to rationalize something the heart had already decided was wrong.
One evening, long after the fire had died and the others were asleep, you found Senku still awake, sketching calculations into the dirt. The moon was just a sliver above you, sharp and cold.
"You’re still working?" you said, voice low.
"No rest for the brilliant," he muttered.
You hesitated. Shifted your weight. "Can I sit?"
He didn't look up. "Free world now. Do what you want."
You sat down beside him anyway, knees brushing his. You smelled like woodsmoke and river water. You were close enough that he could hear the shaky inhale you tried to hide.
"Senku," you whispered, so soft he almost thought he imagined it. "Do you ever think... maybe we made choices just because we were scared?"
His hand tightened around the stick he was using to draw.
He didn’t answer.
He couldn't.
Because if he opened his mouth, even a centimeter, all the wrong things would come out.
Things like Stay.
Things like Pick me instead.
Instead, he scraped the stick against the dirt harder, lines getting messier, jagged.
You laughed — not a real laugh. A noise punched out of your lungs like you were already preparing to lose something.
"I should get back," you said.
He nodded, like he agreed.
Like he wasn't bleeding out in silence.
You stood.
You hesitated.
You looked at him like you were trying to memorize him. Like you were trying to mourn him before you were even gone.
And then you left.
Your footsteps crunched against the dry earth, growing fainter, fainter — until they disappeared altogether.
He didn’t call out.
Didn’t look up.
The moon hung over him, a frozen witness.
Senku sat there until the cold gnawed through his jacket, eating into his skin.
He kept waiting for the feeling to dull, to calcify into something manageable — something he could shelve away under "unsolvable variables" and "acceptable loss."
It didn’t.
It wouldn't.
Somewhere past the river, you were probably curled up beside Tsukasa by now.
Safe.
Warm.
Gone.
Senku dug his nails into his palm, searching for some proof that he was still alive. That he could still keep moving. That he wasn’t tethered to a decision he never even got to make.
The stars burned above him, cold and disinterested.
And in the silence that followed, the only thing Senku could hear was the knowledge that he would keep building this world —
Stone by Stone, life by life —
Without you.
He would make it better.
Smarter.
Faster.
He would save it all.
But even if he built a world vast enough to swallow the stars,
Even if he rewrote the laws of time itself,
It still wouldn’t be enough to draw you into his orbit again.