Crossover Shipgirls: Building a Dimensional Empire from Scratch

The Bulins quickly constructed a smaller port next to the naval base for the five Siren girls. While not as large as the main naval port, it was fully equipped with all the necessary facilities of a standard military harbor.

The design was tailored to the Siren girls’ preferences, featuring a black-and-red aesthetic.

From a vantage point overlooking the city’s northern bay, one could see the two ports standing on either side: the Azure Navy Base on one side and the Siren Port on the other. Both bore the flag of the Starsea Alliance atop their main buildings. However, the outer walls displayed different banners:

the Azure port had the blue naval ensign of the shipgirls, while the Siren port flew the Siren black flag, emblazoned with an emblem resembling a sea monster. Together, they looked like two distinct naval divisions of the same nation.

”Tsk tsk tsk, it really takes you guys to properly develop civilization,” Purifier remarked as she stood in the plaza of the new Siren port, gazing at the imposing main structure.

The black outer walls, adorned with sharp yellow accents, exuded a stern and striking aesthetic that perfectly matched the Sirens’ taste.

”Your Bulins can build ships, repair them, and construct infrastructure. Meanwhile, the lower-tier Sirens back in our world can do nothing but fight,” Purifier sighed. ”This is the first time I’ve ever lived in a proper building.”

The Sirens’ previous existence at sea was akin to roughing it in the wilderness.

”It’s impressive that you managed to push back the port forces so many times under such harsh conditions,” Enterprise commented. Her tone was somewhere between an acknowledgment and a jab.

”Of course! On the sea, we’re not afraid of anyone, not even you!” Purifier shot back, engaging in some playful banter with Enterprise before proceeding to tour their new home, accompanied by the others.

Docks, drydocks, hot springs, dining halls…

The Siren girls were visibly astonished at every turn.

Was this how other shipgirls lived?

Waking up in a comfortable bedroom in the morning, enjoying a buffet breakfast, refueling leisurely, sending the smaller shipgirls to classes while the larger ones patrolled the sea.

After a light training session at midday, they could enjoy tea and snacks in the afternoon, or go shopping in a bustling city. At night, they soaked in hot springs, checked their equipment, and then tucked into soft, cozy beds for a good night’s sleep.

Purifier felt her entire worldview shatter.

”I used to drift aimlessly at sea, baking under the sun every day. If I wanted to improve my meals, I had to use my ship guns to fish. And every now and then, you lot would ambush me! Why?! Why is it that we’re all shipgirls, but you live such cushy lives?!” Purifier wailed.

”Weren’t you the ones constantly causing trouble for the port?!” Richelieu retorted, exasperated.

”If you’d surrendered and joined the port earlier instead of fighting us all the time, everything would have been resolved ages ago!”

”Well, back then, my superiors ordered me to eliminate you…” Purifier scratched her head.

”Now that they’ve gone silent, I guess it doesn’t matter anymore.”

”My directive was to observe the shipgirls and the port’s development. But now I think observing the Commander is more important,” Observer added calmly.

From their names, it was evident what roles each Siren played. Purifier and Omitter were tasked with eliminating enemies, Observer gathered data on the world, and Compiler conducted research and produced Siren units.

”The past is in the past; it’s not important anymore. Let’s all look ahead,” Setsuna said, waving off their concerns.

Although the wars between the two sides had been brutal, thanks to the shipgirls’ resilience, no one had truly perished. This made reconciliation somewhat easier.

”If you need any resources, just submit a request to Yat Sen. With two—no, soon to be three—worlds’ worth of resources at our disposal, supplies are quite plentiful,” Setsuna assured them.

He thought for a moment. With the Siren threat neutralized, the resources of the Azure world could now be fully exploited. Adding Earth and the Demon Slayer world into the mix meant that there were effectively three worlds’ worth of resources at his disposal.

”Living here has one rule: peaceful coexistence. Don’t bully the shipgirls or anyone else.”

”Who’s bullying who here? You’re all starships now. I can’t even beat you if I tried,” Purifier pouted.

”In addition to the shipgirls, there are ordinary people and other species here. More immigrants will arrive in the future. Everyone needs to live in harmony,” Setsuna said as he patted Purifier’s head.

”May I create some subordinate Sirens?” Compiler, usually reserved and quiet, suddenly spoke up.

”There are only five of us here... It feels a little lonely.”

”You can, as long as they don’t randomly fire on people. Ordinary folks can’t exactly withstand your cannons,” Setsuna replied after thinking for a moment. He felt that Compiler was an extraordinary figure, possessing a wealth of Siren-related technologies. However, her cold and taciturn nature suggested it might take some time for this little girl to integrate into the community.

…

For the shipgirls, there was some initial apprehension about their new neighbors.

After all, they had been battling the Sirens on the seas for countless years, with hostilities frequently escalating dangerously close to the breaking point. For many smaller destroyers, the Sirens were little more than terrifying monsters.

If not for repeated assurances from the Commander and several flagship members of the Vanguard Fleet that the Sirens were here solely to cooperate, they wouldn’t have agreed to live so close to them.

But before long, the shipgirls realized the Sirens were indeed different now.

Purifier, in particular, often wandered into the shipgirl base to visit, easily making friends with her outgoing personality despite being a bit ditzy.

Her friendly demeanor quickly won over many of the smaller shipgirls, who began affectionately calling her ”Purifier-chan” as if she were one of their own.

The other four Siren girls, however, remained reclusive, seldom leaving the Siren port. Only occasionally could Compiler or Observer be seen wandering the sea near the city, gazing thoughtfully toward the distant capital.

A few days later, a number of ordinary Siren girls emerged from the Siren port.

Most of them had white hair and yellow eyes, dressed in standard Siren uniforms. Their ship rigs shared a uniform black-and-yellow sci-fi aesthetic.

These ordinary Siren girls organized themselves into small patrol teams, spontaneously patrolling the seas. They even began escorting Bulin supply fleets transporting resources.

”Buli?!” exclaimed the lead Golden Bulin, startled. Sirens used to plunder their supplies, so having Siren girls escort them was a first.

After confirming they meant no harm, the Bulin settled down. Even on a planet without enemies, having more allies around brought a sense of security.

”Explorer, Tracker, Oceana…”

”We’ve crossed paths with these so many times before,” Richelieu muttered, glancing at the sea. Within just a few days, Compiler had created nearly a hundred subordinate Siren girls. The waters near the Golden Horn Bay had grown considerably livelier.

”So harmonious. Almost too harmonious,” Setsuna remarked, narrowing his eyes.

Due to the mass-produced nature of subordinate Siren girls, there were now countless identical individuals mingling with the Bulin workforce. This created a strangely harmonious atmosphere, an image no one could have imagined during the years of conflict between the shipgirls and the Sirens.

Meanwhile, on another front, Akashi had been diligently studying the data provided by Compiler. A few days later, the repair cat approached Setsuna with a stack of documents.

”Commander, I’ve made an incredible discovery about Siren warships. They don’t require any logistical support! As long as they’re in the ocean, they can generate their own energy.”

”The manufacturing process is also extremely simple. For instance, the resources required to build a mass-produced cruiser are only about one-fifth of what’s needed for a Bulin. Of course, battleships and carriers are a bit more resource-intensive.”

”Oh? So cost-effective?!”

The shipgirls were amazed. Bulin units, though simplistic in design and completely devoid of weaponry, still demanded significant resources, especially due to their reliance on cognitive cubes.

”Siren ships also have cores, but their cores are less complex than cognitive cubes. Bulins could likely replicate them.”

Akashi pulled out a black and red polyhedral device— a Siren warship core. It looked distinctly sinister compared to the crystalline elegance of a cognitive cube.

”And what’s their combat effectiveness like?” Richelieu inquired.

”Uh… weaker than shipgirls but stronger than most modern warships?” Akashi paused, considering her answer.

”However, they can operate autonomously. A single mid-level Siren like an Executor-class can control around thirty mass-produced ships at once.”

”Can these mass-produced ships be converted into starships?” Setsuna was more concerned about their adaptability for interstellar use.

”Hmm…” Akashi examined the blueprints before confidently nodding.

”It should be possible, nyan! It’s just a matter of allocating extra resources to replace their propulsion and weapon systems. The core principles remain the same.”

”Excellent.”

Setsuna ruffled Akashi’s ears affectionately.

”Your to-do list just got longer: help the Sirens retrofit their mass-produced ships into starships and establish a new production line.”

”In the future, we’ll need not just a million Bulins for construction but also a million Siren starships.”

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