Kohaku would always find Senku in the lab—an actual lab, now, one that every day approached the level of tech he had used to so long ago—long after all the others had gone to bed, and she'd sit on the table and talk to him about her day while he worked on easy things. He'd listen to her, and when he told her about his day, she'd listen to him.
They weren't in a relationship. They weren't. Senku was nothing if not determined—he would not get distracted. And Kohaku was nothing if not stubborn—she would not deviate from her goals.
But one night Kohaku didn't come to the lab. Senku texted her—the reinvention of pagers was very useful as the tech for sleek smartphones kept leveling up—but didn't receive a response. So he set out on the path he knew she took from her work to his, scanning the area for errant lionesses.
He found her sitting in a tree, looking up at the stars.
"Yo," he called. "Lioness."
She threw her shoe at his head without looking and he barely dodged it. "Not a lioness," she said dispassionately.
She didn't come down, though, and Senku contemplated the tree. Could he climb it?
One way to find out. He jumped up and grabbed a low branch, slowly hauling himself onto it.
"What are you doing?" Kohaku asked in that same tone.
Senku caught his breath. "Climbing…a tree." He scooted to where he could grab a higher branch.
"Why?"
"I don't…want to shout…to talk to you." And it was pretty clear something was wrong. Had he ever heard her be so un-invigorated?
She watched him slowly make his way up the tree, still looking vaguely confused, until he sat on the branch right next to hers. "Now," he said, gasping a little bit, "what's up?"
She sighed deeply. "You're not going to go away until I tell you, aren't you?"
"Obviously." He'd climbed a damn tree to talk to her. He wasn't just going to say hi and leave.
"Someone asked me on a date," she said, and Senku almost fell off his branch.
(What a weird reaction, he thought after a moment. So what? So what if someone else had seen how awesome Kohaku was and wanted a romantic relationship with her?)
"I said no." She wrapped her arms around herself. "But I don't know why. There wasn't anything wrong with the guy, and it's not like I'm dating anyone else."
Senku shifted. "And yet you're moping in a tree."
"I used to sit in trees all the time," she said quietly. "I liked being closer to the stars." She looked at him. "Did you know I love the stars? I don't think I ever actually told you. I do, though." She tilted her head back and looked up at them. "I didn't understand what they were, but even now, they're the most beautiful things in the world to me. I thought…" She trailed off. "You'll think it's the dumbest thing you've ever heard. But I thought my mom, when she died, became a star. I would guess which one was her and pray with all my heart for Ruri to get better." Tears started to run down her face. "But that's not how stars work. They don't know I'm here. They can't know I'm here, because they're just balls of fire or whatever. There's no one there to care or listen."
She said it so sadly and softly that he almost didn't hear it. Was she worried he would make fun of her for that, for her dream of having someone hear and answer her prayer to heal her sister? Of praying when there wasn't anything more she could be doing, when her efforts weren't enough, even as she kept trying, even when she did everything in her power? She knew him better than that, surely.
"They shine and glow, and they'll shine and glow no matter what happens to me." She laughed without any humor. "They're not romantic or dreamy or anything like that. They're just…stars."
Senku had the feeling she wasn't entirely talking about stars anymore. He swallowed. "I thought they were bugs."
"W-what?" She looked at him, confused.
"Yeah. Like, glowing bugs that were just…I dunno, hanging out or something." He smiled and shrugged at her incredulous look. "I was three, okay? I'd seen fireflies. They glowed. So did stars. I saw evidence I thought made sense and drew conclusions from it."
He huffed and went on, "I told my dad I was going to catch some star bugs so I could study the stars up close. Looking back, I think I remember him trying not to laugh, but he helped me plan a little terrarium for star bugs, and it was so much fun." It was one of his favorite memories with his dad now, even if he'd felt silly after he'd figured out that stars were, in fact, giant balls of plasma many light years away and not special glowing bugs.
He smiled as he remembered the week they'd spent making plans for the star bugs before tiny little Senku got excited about something else. "I always thought…" He blinked, surprised at himself. Was he really about to say that? But Kohaku was looking at him, tears still shining in her eyes, and he wanted to. "I always thought I'd do something similar with my kid, when I get around to having one. Plan silly homes for star bugs or…whatever. Whatever weird thing they decided was true for a bit" He shrugged. "It mattered to me. It still matters to me. It's not based on facts, but hearts don't care about facts anyways." He smiled and went on, "And if my dad is anywhere, he's hanging out with the stars."
Kohaku sniffed. "What was the terrarium like?" She sounded curious and not completely dejected.
He almost started explaining but had a thought. "Actually, why don't we make one?"
"W-what? Why? The stars…"
He grinned and started climbing down the tree. "Because it's fun and silly and I feel like doing something fun and silly." And it would cheer Kohaku up.
She laughed but hopped down to the ground. "You, admitting to doing something just for fun? Are you sick or something?"
He paused a few feet from the ground, then started down again. "Nah. Just…a little older now." So what if people knew he wanted to have fun? It didn't make him less of a scientist. And if he wasn't having fun, what was the point? Most of the time, scientific discovery and invention was enough fun to satisfy him, but sometimes he wanted to do something a bit ridiculous.
Like build a terrarium for star bugs. Or, perhaps, for fireflies.
"Let's make a roadmap, lioness!"
He dodged her other shoe. "Not a lioness!"
—
It was probably the most elaborate terrarium for fireflies ever built. More of a greenhouse, really. Taiju and one of his kids thought it was super neat, as did Chrome and his son. The four of them were racing around inside, pointing to the different touches Senku had added.
"But Uncle Senku," said Taiju's kid after a while, tugging on his lab coat, "where are the bugs?"
"That's the very last part," he said. "And only for when the sun goes down."
The kid relayed that information to everyone else, and thus began the rather impatient waiting for nightfall.
There was quite the group gathered in the greenhouse once the sun set, once the last of the light was fading from the sky. "We wanted to see what you've been up to!" Ryusui said with a laugh. "A personal project of Senku's? It must be incredible!" Kohaku walked up then, a huge barrel in her arms. "And a collaboration, no less! It will be a sight to see. Am I wrong?"
Senku chuckled. "No, it will be ten billion percent worth the wait." Considering the placement of the sun, he nodded to Kohaku. She put the barrel in place and opened it into the terrarium.
Slowly, hundreds of fireflies emerged from the rather complicated barrel keeping them safe.
The kids loved it immediately, swarming around.
As soon as the fireflies started landing on the various perches and walls, the adults started gasping, too.
The glow of the fireflies was causing the various chemicals Senku had painted around to glow as well—and what they were left with was something that looked like a moving, living galaxy, with stars lazily flying around, clustering together, swirls and patterns and single lights twinkling and flashing.
I finished it, Dad. I wish you could see it.
"I wish my mom could've seen this," Kohaku said from the spot she'd taken up by his side. "All of it. What science can and does do." She nudged her arm against his. "But this is how we keep them alive in our hearts, right? Living. Making things. Thinking of them."
Their friends were circling the terrarium with their families. Chrome had his little girl on his shoulders, and Ruri was carrying their baby as their son ran around. Taiju was now wrangling two of his sons while Yuzuriha followed sedately with their toddler. And more and more—their friends were living their lives, growing families and making more friends.
And Senku realized that now was the moment. "Kohaku," he said, "I've got our next project ready to go."
She laughed. "Never even a moment of rest, huh?" Her eyes were shining, though. "Well, Dr Stone, what will you need my help with?"
"Don't call me that," he said with an eye roll. "You should know that it's a project that will take the rest of our lives to complete."
Kohaku snorted. "Well, if it'll take so long, we should start right away."
"It'll probably require a change in living arrangements."
"As long as I'm able to visit my family, I can be anywhere."
He put his hands on his hips. "We'll be spending a lot of time together through the whole thing. And we might end up with a few extra people on the way, but we'll have to start with just the two of us."
She nodded, then raised an eyebrow. "Are you going to tell me what it is we'll be doing?"
Senku held out the little box he'd grabbed from his pocket, popping the lid open. "Being a family. If you want to."
She looked at the ring with wide eyes. She knew enough about his time to know what offering a ring meant. "But…but we…"
"Yes?"
"We're not even dating!"
"Do you feel like we need to? We could start there instead." The best plans were adaptable to circumstances.
Her lip trembled, and then she laughed and wiped her eyes. "No, I think we know each other well enough to know how this will go." She grabbed the box and took out the ring, sliding it onto her finger and then admiring it. "Okay," she said with a growing smile. "Let's start a family together. Let's get married."
And there was something about seeing her smiling face in the bioluminescent light, about knowing that she was choosing him, to build a family with him, that made him want to do something rather impulsive. So he gently held her chin in one hand and bent down to kiss her. He could feel her smiling into the kiss.
Which is when Nikki saw them and literally gasped, grabbing onto the people nearest to her and pointing, which led to their quiet, lovely little moment being interrupted by nearly all their friends swarming them with questions and exclamations, then congratulations and planning for probably the largest wedding this new world had ever known (neither Senku nor Kohaku were happy about that). But Kohaku didn't leave his side and the fireflies kept flying, the stars twinkling in tandem far overhead.
