Crossover Shipgirls: Building a Dimensional Empire from Scratch

”Hm, looks like things in the Konosuba world are mostly settled now.”

After returning from the lunar research base—and stopping by several lesser goddesses’ cathedrals along the way—Setsuna checked off another item on his to-do list.

All the rescues he had planned across the other worlds were complete.

The Konosuba Squad was now adventuring regularly with the Demon Slayer Corps, Night Raid, and Goblin Slayer teams. The Crimson Demon girls were studying diligently at the Imperial Magic Academy.

The goddesses, meanwhile, had taken quite a liking to immigration. Every so often, they would file official petitions to relocate to the Starsea Empire, hoping to entrust their worlds to the Sophons while developing new followers here.

After all, the empire’s territories and population dwarfed those of any single isekai world. And as expansion continued, it was clear there would be endless new worlds to conquer.

To the goddesses, every new world represented tens of billions—if not hundreds of billions—of potential believers.

Life for the goddesses in the Starsea Empire was exceedingly comfortable. Many who had struggled to survive before were now enjoying the full privileges of divinity for the first time.

Each one of them swore that if the Commander ever needed them, they would give their all—those who could fight would join the frontlines, while those less suited for combat would stay behind to provide divine buffs.

All they asked in return was to be allowed to preach their faith and sip a bit of divine ”soup” during future world expeditions.

As for that idiot Aqua?

She was still at the lunar research base, being doted on and teased endlessly by the research lolis and Grey—treated more like the department mascot than a goddess.

Every day, they pulled her into new experiments to analyze the power structure of divinity.

Some shipgirls even planned to wait until World Aqua’s divine power grew stronger before taking her along for interstellar terraforming projects.

”Uh, according to reports from the Starbase Division, ever since the goddesses arrived, their overall efficiency has improved significantly,” said Yat Sen as she read through a stack of papers.

”Many projects that used to require multiple test cycles now succeed within just two or three attempts—their data readings are practically flawless.”

”Even prototype experiments with success rates below 10% somehow end up producing the best possible outcomes in the strangest ways.”

”The Research Division suspects this might be because some of the lolis have begun worshipping Eris. They’re even considering building a church for her at the Starbase—so they can pray before running tests.”

”For major experiments, they also plan to invite her to give her blessings personally…”

”That’s great news,” Setsuna said with a chuckle.

In ordinary circumstances, scientists praying to a goddess would sound like superstition.

But this was different—Eris was literally there, tangible and visible, and her blessings produced measurable results.

Back in the early days, Elma had once suggested that the Starsea Empire should adopt a guardian deity—a divine protector whose authority could bring subtle advantages. They had considered several candidates at the time.

Now, it seemed the goddesses from the Konosuba world could easily fill that role for the moment.

Of course, in time, a higher god would come—one strong enough to unify them all under a single divine banner.

”...”

Once the Konosuba world was fully stabilized, Setsuna shifted his focus back to domestic affairs.

He first visited the Siren Naval Port.

The black-and-yellow harbor was filled with Executor-class Siren girls, who usually stayed near the Experimental Division under semi-autonomous control.

They rarely visited the capital or interacted with others, focusing entirely on assisting with the Experimentals’ operations.

In a vast open testing ground, Observer and Purifier were supervising the Bulins as they constructed a strange, massive device.

Upon seeing Setsuna, both floated over with their riggings deployed.

”Hmm, Commander! Long time no see… well, actually, not that long?”

”I have two pieces of good news—one is normally good, and the other is really good! Which one do you want first?” Observer said cheerfully, waving her mechanical tendrils.

”...Let’s start with the normal one,” Setsuna sighed.

”We received another transmission from the Arbiters a few days ago,” Observer said. ”They’ve successfully located the spatial beacon we built here in the port.”

”The Arbiters are preparing to cross dimensions. Once their systems finish charging, they’ll traverse space and arrive here. We should be seeing them very soon.”

”...”

”Oh? Then I’ll notify the port ahead of time to station a welcoming team?”

Setsuna suggested.

In the Azure Lane world, the port area had long since turned into a resort. Shipgirls and others often spent their downtime there sunbathing or surfing for fun.

If an unknown fleet suddenly appeared out of nowhere, it could easily cause misunderstandings.

”No need~” Observer waved her tendrils again, flashing a sly grin.

”Leave the welcoming to me and Purifier. We’re going to give the Arbiters a little surprise.”

”Tch, they’ve always looked down on us—dumping all the dirty work on us, like building test fields and observation zones. But now? The Experimentals are the strongest faction in the entire Siren Fleet!”

”We’ve accomplished something the Arbiters never could—we went to space! Faster-than-light travel! We built super starships!”

The little Siren grew more excited as she spoke, her rigging’s tendrils waving energetically behind her.

In the past, the Arbiters had been the dominant branch of the Siren system, wielding the most advanced technology.

But after witnessing the magic and technology Setsuna had brought from other worlds, the Experimentals realized they could now achieve things their counterparts never could.

Observer and Purifier were absolutely ecstatic.

”When the Arbiters arrive, we’ll be there to welcome them personally—scare the circuits out of them.”

”Maybe Dreamweaver will even promote us! If they can be Arbiters, why can’t we?!”

Purifier was practically dreaming of rebellion already.

”...Fine,” Setsuna agreed, humoring their request.

”As long as you’re happy. Just don’t cause trouble when your colleagues arrive…”

”So what’s the second good news?” he asked.

”Well…”

Observer brought up a holographic screen.

”We’ve made a breakthrough in dimensional weapon research. Originally, we had over three hundred theoretical models to test—planning to try them one by one until something worked.”

”But then I went to pray to Eris for a blessing, picked one at random, and it succeeded instantly.”

”...”

Setsuna silently marveled—apparently, the Goddess of Luck’s value was still climbing.

”So now you’ve developed a working controllable dimensional weapon?”

”Hmm… not exactly. The ’dimensional strike’ concept is only one aspect of dimensional technology. We’ve gone beyond that.”

Observer brought up a series of data displays before Setsuna.

”Commander, dimensional strikes are the simplest and crudest use of dimensional science—collapsing higher-dimensional space to destroy targets in lower dimensions.”

”Simple, convenient, and low-cost—but highly contaminating, right?”

Setsuna nodded.

Dual-Vector Foil technology was, after all, an effective cleanup tool—cheap, irreversible, and catastrophic if overused.

”When studying dimensional technology, we decided to explore a different path.”

”Instead of reducing the enemy’s dimension to destroy them, what if we enhanced ourselves—in other words, raised our own dimensional level?”

Compiler floated over to join the discussion, her holographic body shimmering.

”Huh?”

”Enhance ourselves?” Setsuna blinked in surprise.

Siren technology often had a way of catching him off guard.

”You mean… turning into four-dimensional beings? Like something out of a cosmic fantasy?”

”No, no, Commander—think bigger,” Observer said, suddenly taking on the tone of a physics professor explaining frontier science.

”In higher dimensions—say, six or seven—time itself becomes a dimensional component, and it doesn’t have just one direction.”

”That means, for a higher-dimensional lifeform, it can move freely between past, present, and future—appearing anywhere along a timeline at will.”

”Oh…” Setsuna exhaled slowly, finally piecing together her meaning.

He remembered how the Zero-Homer had once described the eleven-dimensional universe of the Elysium Era—where time had multiple directions, and countless choices could exist simultaneously until the optimal one was realized as reality.

”That sounds amazing—but we’re three-dimensional beings. How would you make us ascend to higher dimensions?” asked Gudako, scratching her head.

”Upgrading biological organisms directly isn’t realistic,” Observer admitted. ”But we can try something else.”

She leaned forward, eyes gleaming.

”Commander—you’re already synchronized with the Eternal Snowfall and Cattleya. That means we could perform a dimensional enhancement on a starship or mobile armor instead.”

She made the proposal without hesitation.

”Hmm… a higher-dimensional ship? Like a Sophon?”

”No. A Sophon is just a microscopic particle—it can barely influence reality. Raising a particle’s dimension isn’t the same as elevating an entire ship.”

Observer shook her head.

”Commander, after the upgrade, everything in the three-dimensional world would be transparent to you. More importantly, you’d gain a remarkable ability—the power to move freely through higher-dimensional space.”

”Without relying on spiritron transfer devices, you could travel freely along the timeline—not just to the past, but…”

”To the future of a world.”

”!”

Setsuna snapped his fingers.

”...Travel to the future? I see exactly what that could do.”

”Get started right away.”

...

Solar System Base — Starship Dock.

Setsuna materialized one of the battleships carried within the Eternal Snowfall.

The Juggernaut-class flagship was equipped with numerous sub-vessels—battleships, cruisers, and destroyers—designed for planetary operations and tactical mobility. The vessel before him was one such ship, its hull marked with the code —001.

”Alright, next up, we’ll conduct the higher-dimensional modification experiment on this sub-ship of the Eternal Snowfall.”

Observer clapped her hands sharply, leading a group of Bulins as they towed the ship into the dock.

”Commander, this is our first time applying higher-dimensional technology. Directly modifying the Eternal Snowfall’s main body would be far too massive an undertaking, so we’ll start small.”

”The results will be the same. Once this test ship is successfully elevated in dimension, you’ll be able to use it to travel freely across the timeline of a world.”

The little Siren waved her tendrils enthusiastically as she explained.

”Sounds good.”

Setsuna’s attention was fully fixated on one feature—traveling to the future.

Back during his time in the Honkai world, he had long pondered how to deal with the gap between the previous civilization and the current one.

Traveling to the past had been one thing—constructing a massive spiritron transfer array, burning through star cores and Wisdom Cubes if necessary, brute-forcing miracles through sheer energy output.

But traveling forward into the future had stumped even the Research Division for ages. No one could devise a reliable method.

The Once Upon a Time in Shenzhou timeline lay hundreds of years ahead—he couldn’t possibly wait centuries to act.

Of course, in theory, he could. Thanks to genetic enhancement, super-evolution surgeries, and shipgirl modifications, lifespans across the empire were virtually limitless—time was the one thing they didn’t lack.

But Setsuna wanted a more efficient way.

And now, with the breakthrough in dimensional technology, the obstacle to reaching the current civilization’s era had finally been solved.

”Our plan is to upgrade this vessel to six-dimensional structure—four spatial axes and two temporal axes.”

”If all goes well, we’ll then attempt to modify the Eternal Snowfall’s main body.”

”Commander, this will be one of the greatest technological milestones in Starsea Empire history! Its significance rivals faster-than-light travel—no, it might even surpass it!”

Grey and Observer were practically glowing with excitement, eagerly explaining the details to Setsuna and the nearby shipgirls. The flood of technical jargon left most of them dizzy.

But it was clear how much this project meant to them.

”Once, we broke the light-speed barrier. Now, we’re about to transcend dimensions!”

”From superluminal ships… to transdimensional starships!!!”

”...”

Setsuna wasn’t a scientist, but he understood enough to grasp how outrageous this new technology was.

A transdimensional vessel would possess absolute advantage over any three-dimensional target.

Time-jumping along a worldline was only its basic function. There were countless other potential effects—most still theoretical—awaiting validation once the prototype was complete.

”Ahem. I’ll leave the rest to you. Whatever resources or support you need, just contact me.”

”Finish the transdimensional ship as soon as possible. I’ve got people waiting to be rescued in the future.”

After giving his orders, Setsuna dismissed the team and summoned a shuttle for his return trip.

He reached down to ruffle Observer’s hair—and, on a whim, pinched her cheek.

”!”

The little Siren bared her teeth and chomped his hand in protest.

”Commander! You’re the worst!!!”

”You’re leaving all the fun to us again while you go enjoy yourself, aren’t you?!”

”How’d you know?”

Setsuna smirked.

Every time he returned from an expedition, he made sure to indulge in some well-earned relaxation.

And since this project didn’t require the Eternal Snowfall’s main body, the silver party mode could continue as planned.

”Where are you going this time? Take me with you!”

”You’re not planning to go on another date with one of the shipgirls and do something indecent again, are you?!”

Observer puffed her cheeks, pouting adorably.

”I’m hosting a party. You sure you want to come?”

Setsuna nearly laughed.

”Wha—?!”

The little Siren’s face instantly turned red.

She might have no sense of shame most of the time, but faced with a direct tease, she was still no match.

”Wait, that’s not what I—!”

”Then take me with you! I want to observe your activities.”

Observer’s eyes gleamed mischievously as she tilted her head up, trying to sound serious while clearly scheming.

”For instance, to monitor whether humans and shipgirls can reproduce naturally—and to record data such as the Commander’s… duration and intensity of activity…”

”It’d also be best to document which positions the Commander prefers. I’ll report everything truthfully to Dreamweaver and the Arbiters…”

”I am the Observer, after all! It’s my job to monitor every detail about you!”

”?”

”Wait—since when was that part of our mission parameters?!”

Purifier looked baffled, scratching her head. ”I don’t recall Dreamweaver ever assigning us that kind of task…”

”You talk too much. Go sketch some diagrams.”

Observer’s rigging extended a single tendril, wrapping around Purifier and hurling her toward the Research Division in a perfect arc.

”Ooooohhhhhhh—!!!”

The research lolis watched as the white-haired Siren, ponytail fluttering, streaked across the sky like a meteor before crashing headfirst through the Research Division’s roof.

”...”

Setsuna stared at the Siren for a long moment.

Among the five Experimentals, Tester and Omitter showed little emotion, Purifier was a hyperactive idiot, Compiler was quiet and reserved—but this one, this little gremlin, spent half her time teasing shipgirls and himself for fun.

And what was the correct way to handle a mischievous little gremlin again…?

Smack—

Setsuna grabbed Observer in one motion, tucking her under his arm like a bag of loot.

With a sharp click, he opened the hatch and tossed the squirming loli inside.

”I’m borrowing Observer for a couple of days. You all keep working.”

...

”Yo, a rare guest?!”

Aboard the Eternal Snowfall, several shipgirls looked up in surprise as Setsuna casually tossed Observer into the hangar.

”She said she wanted to tag along and see the sights,” he said with a yawn, wrapping an arm lazily around Kurumi and pulling her close.

”My dear husband, one moment please…”

Kurumi gently pressed a hand to his chest, smiling faintly.

”I thought Lady Eris would be the next to come aboard, Master. But it’s her?”

Jeanne Alter folded her arms, giving Observer an unimpressed look.

Those who had been with Setsuna the longest—the earliest members of his fleet and harem—often made bets about which girl he’d bring aboard next.

After all, joining the Eternal Snowfall practically meant being added to the Commander’s inner circle.

After his trip to the Konosuba world, most of them had assumed he’d eventually bring one of the goddesses. Aqua was far too ditzy—forever the research division’s plaything—so she was out. Eris, on the other hand, was gentle, kind, and clearly favored by him… she seemed the perfect candidate.

But instead, he’d brought back… a Siren.

”...Are you all plotting my future conquests now?” Setsuna sighed, rubbing his forehead.

”Well, it’s not a bad suggestion,” he added with a smirk. ”I do like Eris.”

”...”

Observer shivered.

She glanced around—the shipgirls were all familiar faces, with a few Spirits and humans mixed in.

Seeing Setsuna lounging among them, arms draped around multiple women, she quickly pieced things together.

”Richelieu, Taihou, Shoukaku… wait—are you the Shoukaku from the Kancolle port? Illustrious, Nagato, Tohru…”

The little Siren began counting on her fingers, then realized she didn’t have enough—so she started using her rigging’s tendrils too.

”Hentai Commander!!!”

”There’s no way—you can’t possibly handle this many at once!!!”

Her imagination was clearly failing to keep up.

She couldn’t even comprehend what kind of chaos Setsuna must be causing up here.

”Aren’t you Observer?” Setsuna teased.

”Your role is to analyze the subjects you observe, gather their data, and upload it to the Siren database, correct?”

”Y-yes…” the little Siren nodded automatically.

”Then don’t you know there’s a more efficient method than passive observation?”

”Eh? What method?”

”Simple,” Setsuna said with a grin. ”Experience it firsthand.”

—

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