Crossover Shipgirls: Building a Dimensional Empire from Scratch

Building a Star Destroyer was obviously just a joke.

While a starship would be far more powerful than a Gundam, constructing one wasn’t feasible at the moment. In the future, when starship-class weapons become available, Setsuna could simply mount them onto a Gundam. After all, he had absolute confidence in his body’s compatibility.

The multiverse offered an endless array of incredible technologies and magic. There was no need to limit oneself to a single approach.

Style, after all, was a lifelong pursuit.

The Gundam had caused a huge stir among the destroyer-class shipgirls. They were utterly fascinated by this massive, versatile machine capable of soaring through the sky and dominating the battlefield.

Every day, they either pestered Setsuna to ”fly high” with them or swarmed Akashi’s workshop, begging to borrow a mech for a test drive.

The constant pestering drove the repair ship into a frenzy.

”Aren’t you all shipgirls?! Why are you so obsessed with piloting mechs?!” Akashi shouted in frustration.

”What’s next? We deploy the Gundam in battle, and when it explodes, it ejects a shipgirl from the cockpit?! That would be way too surreal!”

Two weeks later, at dusk.

In the naval base’s office, Setsuna and Richelieu were going through their paperwork.

”Commander, Ning Hai and Ping Hai have sent a report. The first batch of resources from the Demon Slayer world is about to arrive,” Richelieu said.

”It includes iron, steel, coal, non-ferrous metals, various agricultural products, and textiles. The quantities are staggering. We might need to dispatch additional Bulins to transport everything.”

Ning Hai and Ping Hai had meticulously calculated the tribute resources from each nation.

For industrialized countries like the British Empire and France, the tribute focused on refined minerals. For agricultural nations, it centered around produce and textiles.

”Refined steel: 7 million tons. Coal: 54 million tons. Tin: 60,000 tons...”

Even a glance at the report was enough to leave someone speechless at the sheer scale.

This shipment alone far exceeded the output of this world’s industries, perhaps by dozens or even hundreds of times. And this was merely the first batch. The resource extraction ratios were intentionally conservative to avoid disrupting the Demon Slayer world’s development. Once industrialization was complete and the entire world advanced into the industrial age, the returns would be astronomical.

”Send some of these resources to the port district,” Setsuna instructed. ”They’re constantly running experiments and building new weapons. I’m sure their supply lines are stretched thin.”

”Also, tell Enterprise that if they need anything, they can requisition it from here. I’ll personally cover all expenses for the shipgirls.”

Richelieu chuckled. ”You’re quite generous, Commander.”

”What about us? Should we invest in expanding the capital’s construction? Or perhaps produce more torpedoes and carrier planes for military use?” Richelieu asked.

”... Stop, stop, stop,” Setsuna interrupted, tapping his fingers on the desk. ”Our warehouses are already overflowing with military stockpiles. We have enough weapons to last several wars.”

”If it weren’t for the risk of destabilizing the Demon Slayer world, I’d have already exported a batch of torpedoes.”

”Then where should we allocate the resources?”

”All of it goes to producing Wisdom Cubes and building Bulins.”

”... Huh?” Richelieu blinked, perplexed.

Setsuna elaborated, ”Remember when I absorbed a Wisdom Cube back at the port? Akashi mentioned that the lack of resources prevented them from producing more Cubes.”

”Now that we have an entire world’s worth of surplus, it’s only logical to start mass-producing Bulins.”

Setsuna continued with an analogy. ”In any strategy game, the early stages are all about developing your economy. The priority is producing miners and transporters while ensuring your base is secure.”

”The more miners and transporters you have, the faster you can expand into heavy industries, mass-produce troops, and construct advanced research facilities.”

”If you blow all your resources on tanks to attack your neighbors early on, you might gain a short-term advantage, but you’ll lose in the long run because you can’t sustain growth.”

”Our city doesn’t have a population cap. The more miners and transporters we have, the better. It’ll even speed up building wonders later.”

Richelieu spent a moment processing Setsuna’s ”miners and transporters” theory.

Eventually, she nodded in understanding. Bulins indeed played a role analogous to versatile workers, and perhaps even exceeded them. Not only could they mine and smelt materials, but they could also repair warships and manufacture advanced munitions for shipgirls. Even Setsuna’s Gundam had been assembled by Bulins wielding spanners.

Having more Bulins would undoubtedly accelerate the city’s development.

As for the small yellow chicks... well, they were only good for hauling goods and farming. For industrial tasks, Bulins were irreplaceable.

Five minutes later.

Richelieu dragged a lounging Akashi into the office.

”Commander, are you finally going to pay me back for the Wisdom Cubes and the Gundam?!” Akashi exclaimed, producing a ledger. ”The total comes to 1.6 billion! But since we’re on such good terms, I’ll give you a 5% discount!”

”Wait, building the Gundam cost money?! You didn’t mention this before!” Richelieu groaned.

”Don’t worry,” Setsuna said. ”I’ll settle the account—if Akashi helps me with something first.”

He showed her the resource list.

”How many Wisdom Cubes could you produce with all of this?”

”Th-this much?!” Akashi’s eyes sparkled as she quickly did the math. ”About 8,000 Cubes, meow...”

”And if we focus on producing Bulins, factoring in the ship construction costs, how many could we make?”

”Uhh...” Akashi twitched her ears and recalculated. ”With a mix of special, prototype, and Universal Bulin... roughly 6,000, meow.”

”Perfect.”

Setsuna patted Akashi on the head. ”Use the resources to produce 6,000 Bulins. Consider the remainder my payment to you.”

”... Meow meow meow meow?!?!”

At dawn, as the first rays of sunlight painted the ocean’s surface, a massive fleet slowly entered Golden Horn Bay from the open sea.

At the head of the formation were two light cruisers in distinct Dragon Empery style, and trailing behind them were over a hundred transport ships resembling ocean liners.

Each liner looked almost identical, and standing on the deck of each one was a purple Bulin.

Although Bulins had their own ship bodies, they lacked combat capabilities, making them ideal for transport missions.

On the bridge of the lead cruiser, Ning Hai and Ping Hai stood, gazing at the sprawling city on the horizon.

”It feels like it’s grown even larger than when we left for the Demon Slayer world,” Ning Hai said, squinting at the skyline of Constantinople, now known as the capital of the Starsea Alliance.

Indeed, compared to when they had first departed, the city’s construction had progressed significantly. Buildings had sprung up on a massive scale along the central thoroughfare, flanked by residential areas, commercial zones, and research districts.

Drawing inspiration from the ancient eastern dynasties, Setsuna had designed the city with flying eaves, intricate corbel brackets, and glazed tiles. With the advanced technology of the Azur Lane world and the tireless efforts of the Bulins, the city combined classic elegance with fully modernized infrastructure.

The only downside was the low population density, making the grand cityscape seem somewhat empty.

”Demon Slayer World First Fleet, returning!”

From the watchtower, a Bulin on guard duty spotted the incoming fleet and promptly cleared the port’s waters to allow their arrival.

The Bulin fleet, laden with containers, entered the harbor, and the yellow chicks swarmed over to assist with unloading.

”This ship’s cargo is iron ore from England, and that one has animal hides and timber from African chieftains, burin!”

”There’s also beef from South America, red wine from France, and kangaroos and koalas from Australia, burin!” a purple Bulin announced with her hands on her hips, enthusiastically introducing the cargo to the gathered shipgirls.

The resources brought back from the Demon Slayer world were incredibly diverse. Some were tributes offered as part of sustainable exploitation agreements, while others were voluntary gifts from nations eager to curry favor with their mysterious overlord, who had introduced them to advanced technologies.

The sheer scale of the haul was staggering. Just coal, iron, and steel alone were measured in millions of tons at a minimum, with some exceeding tens of millions of tons. While the Demon Slayer world’s technological level was low, its collective resource extraction capacity was impressive.

”Is this... a tribute from all nations?”

Yat Sen, observing the endless stream of cargo being unloaded, felt as though she had been transported back to the ancient times, standing in the capital and receiving emissaries from across the land.

”Forget the wine and beef—what are we going to do with koalas and kangaroos?! Are we supposed to box with them?!”

”And these tiger-skin skirts—what’s the deal with those?! Are you planning to make the Commander a tribal chieftain?!”

The ever-curious Formidable pulled a tiger-skin skirt out of a box, her expression somewhere between confusion and amusement.

The crude garment looked like something a tribal leader would wear after a successful hunt—definitely handmade, judging by the craftsmanship.

”This is probably the best they could offer,” Ning Hai scratched her head.

”Once the railways are in place and they’ve been taught modern farming, they should be able to contribute something more useful.”

Given the limited time Ning Hai and Ping Hai had spent in the Demon Slayer world, the varying levels of development across its nations made such eclectic tributes understandable.

”Good work,” Setsuna said, patting the sisters on the head as part of his welcome.

”Strategic materials will be sorted and handed over to Akashi for processing. We’re going to need them for our next population boom.”

”As for the miscellaneous items, just store them for now. Oh, and open a few bottles of wine tonight to celebrate.”

”Hehe!” Ning Hai and Ping Hai giggled at the suggestion before remembering something important.

”Right, Commander. On our way back, a few people insisted on tagging along.”

”They said they wanted to thank you in person because you saved their lives. So, we brought them with us.”

From the hold of Ping Hai’s ship emerged three somewhat familiar girls.

Leading them was a young woman in a dark uniform, draped in a haori with butterfly patterns. Behind her were two younger girls, one with short hair and the other with a side ponytail.

All three wore butterfly-shaped hairpins and carried long swords at their waists.

The Butterfly Sisters: Kanae Kocho, Shinobu Kocho, and Kanao Tsuyuri.

”Oh, it’s you three?” Setsuna said, surprised.

After Muzan was defeated, he had been too preoccupied with conquering the Demon Slayer world to follow up on the Demon Slayer Corps.

”When we fought Doma, you and your companions saved my life just in time,” Kanae said, bowing deeply.

”But we never had the chance to thank you properly afterward. For that, I sincerely apologize.”

Shortly after that battle, Setsuna had embarked on a bold gambit to trick Muzan, a plan so swift and chaotic that even the Demon Slayer Corps couldn’t keep up. The demon king was conned so thoroughly that he didn’t know what hit him before a hydrogen bomb sent him to oblivion.

After that, the fleet toured the world, leaving no opportunity for anyone to catch up.

”Thank you for saving my sister!” Shinobu and Kanao followed suit, bowing deeply.

Kanae then retrieved a sword wrapped in cloth and presented it to Setsuna with both hands.

”This is the finest Nichirin Blade ever forged by the craftsmen of Swordsmith Village. It may not mean much to you, but it’s the best gift we can offer.”

”This blade carries the power of sunlight, capable of slaying all evils and ensuring your safety.”

Setsuna accepted the Nichirin Blade.

It felt solid in his hands, and its design was noticeably more refined than the standard issue of the Demon Slayer Corps—a testament to the smith’s dedication.

Click—

As he drew the blade, its indigo steel gleamed, etched with the characters for ”Destroyer of Demons.”

”Thank you,” Setsuna said. While he didn’t need a Nichirin Blade to deal with threats—Strong Strike Freedom could pulverize anything into paste—the sentiment behind the gift was deeply appreciated.

”What happened to the other members of the Demon Slayer Corps?” Richelieu asked casually.

”With Muzan gone, the demons have been completely eradicated. With nothing left to do, most members either traveled the world or returned to farming and fishing,” Kanae explained, glancing curiously at the bustling port and city around her.

The advanced naval base, grand harbor, and distinctly Eastern-style cityscape were unlike anything she had ever imagined.

But what truly baffled the Butterfly Sisters was something else entirely.

”Um... Mr. Setsuna, there’s been something on my mind since earlier...” Kanae hesitated.

”Why... are there chickens driving tractors and farming here?!”