Crossover Shipgirls: Building a Dimensional Empire from Scratch

”Confirming: the host will simultaneously initiate the Galactic Nemesis / Cosmogenesis ascension route.”

”Please further enhance your civilization’s strength to unlock the corresponding ascension route’s special technologies.”

Shuaa—

Unexpectedly, the system didn’t force him into a binary choice.

Instead, it dumped another pile of data into the Eternal Snowfall’s database.

A brief glance revealed two sets of preliminary theories and related technologies for the Ascension Perks.

For example, theories on synaptic condensation engines, frame blueprints, and other bizarre concepts.

It was equivalent to holding an entry ticket to the Ascension Perks.

”...I’m almost not used to this anymore.”

Setsuna muttered.

Most of his spoils were obtained from other worlds through personal combat—it was rare to receive a direct reward from the system.

Still... whether it was Galactic Wonder or Ascension Perk, there was no doubt about their immense power.

”This is meant to further enhance the host civilization’s overall strength—to face higher-level challenges.”

The system responded.

”Challenges? Like... fighting crisis?”

Setsuna blinked.

”After nearby worlds have been conquered, in farther reaches of the multiverse, you will face new worlds with even greater challenges.”

The response was, as always, devoid of emotion.

”Tsk, so basically you’re just cranking up the difficulty, huh?”

He got it—after defeating all the easy neighbors nearby, the time had come to fight higher-level worlds.

”Well, fine. The greater the risk, the greater the reward. Aside from stealing girls, there’s not much worth looting from the natives anyway.”

”Guess it’s time for a little exercise.”

Even if he couldn’t win immediately, he could always hole up at home farming resources—or head to another world to think of a plan.

During all of his interdimensional expeditions, Setsuna never fought an unprepared war.

...

Under Setsuna’s command, the Starsea Empire once again entered an era of fervent large-scale construction.

Beyond Jupiter’s orbit, millions of colorful Bulins piloted their small ship-bodies, assembling a gigantic framework in space.

Even its basic structure rivaled the size of Earth—on an astronomical scale, it resembled a massive umbrella-shaped radar.

The Sentry Array.

To map out the surrounding regions, this exploration wonder had to be built first.

Outside the Starbase, countless Angeloids constructed an array of orbital platforms above the blazing sun.

Since the Solar System was the Empire’s homeworld, building a Dyson Sphere would freeze the entire system solid.

A lighter version—the Dyson Swarm—was perfectly feasible.

According to the plan, millions of energy arrays would be anchored to the sun’s surface, drawing colossal amounts of power from the star.

Beyond the gravity well, structures similar to Gateways were also under construction, designed to link the homeworld with star systems thousands—or even tens of thousands—of light-years away.

Together, they would form the Starsea Empire’s high-speed interstellar transport network.

Farther into deep space, expeditionary Bulin engineering fleets were heading toward the nearest black hole to construct the Matter Decompressor anchor station.

From orbit, shipgirls aboard passing vessels could occasionally glimpse those vast construction sites.

Even though the projects were only in their early stages, their colossal frameworks already hinted at the breathtaking grandeur the Galactic Wonders would display upon completion.

...

A few days later—

”Ah, this is nice~”

”If only we could build a Sentry Array and a few Gateways here too~”

In Azure Lane’s port district, Observer lay lazily on a beach chair, sipping orange juice and basking under the sun.

The massive construction projects in the main world hadn’t disturbed this place at all.

Ever since the shipgirls and Sirens collectively moved to the capital, the Azure Lane world had become a vacation planet.

The port area underwent major renovations—the outdated defense system was completely overhauled, and various entertainment facilities were built.

The old shipyard factories were converted into cinemas, while the docks were transformed into water parks for little girls and small shipgirls to play in.

Everywhere along the beach, Angeloids and Bulins sold barbecues and cold drinks.

Many shipgirls and girls were laughing and playing by the waves.

Illustrious, Victorious, and several other carrier shipgirls were playing beach volleyball not far away, drawing everyone’s gaze not only to the ball—but to the lively movements beyond it.

Though this world was technically the shipgirls’ home, they didn’t mind sharing it—as long as it was a fellow shipgirl from the base, everyone was welcome.

”Tsk, I figure those wonders will take a few years to finish. If we throw in enough Bulins... maybe two or three years?”

Purifier lay nearby, lazily twirling her hair.

”Two or three years to build a Galactic Wonder? That’s already Bulin power at its finest!”

Observer kicked her little legs idly.

”Hmm, but Commander’s been so busy supervising those projects lately... he hasn’t spent much time with me. Such wonders—ruining nations and breaking hearts.”

”...”

Purifier squinted disdainfully at her.

That coworker of hers often snuck aboard the Eternal Snowfall, only to return with heart-shaped pupils and shaky legs after being thoroughly ’educated’ by Setsuna.

Typical lusty little goblin.

”But speaking of which... isn’t Empress and her crew supposed to arrive soon?”

”If they don’t get here soon, I’m heading back to work at the Starbase. Compiler’s already bugging me...”

The two Siren girls chatted casually—

Until suddenly—

Vmmmm—

A deep spatial vibration resounded in the distance, and the skies above the port instantly darkened.

Just moments ago, it had been cloudless and bright. Now, storm clouds gathered overhead.

Rumble—Rumble—

Thunder echoed faintly from high above.

”Eh?”

The girls sunbathing, grilling skewers, or petting catgirls froze in surprise.

They stopped what they were doing, looking up at the sky in confusion.

”Hey! Who’s messing with the weather controller?! Or is someone experimenting with climate-altering magic again?!”

”Fu Shun, was it you?! Or Chang Chun?!”

Formidable looked around suspiciously.

”Not me! I’m in co-op mode with Fu Shun right now!!”

A white-haired loli shipgirl with a tiger hat shouted indignantly.

”Huh? Then who’s—”

Before she could finish, the temperature across the port seemed to drop several degrees.

Crack—!

A sharp shattering sound echoed through the air.

High above the sea, thousands of meters up—

[Target universe located. Coordinates confirmed. Commencing descent.]

Ka—!

Space fractured like a mirror.

From the void emerged a vanguard fleet—hundreds of ships, painted in shades of blue and black.

They were roughly the size of small destroyers, blue flames blazing from their engines. Their sleek, alien designs bore resemblance to the Experimentals’ mass-produced ships.

The vanguard formed an escort formation, followed by a massive armada of Siren warships pouring through the rift.

Their numbers easily exceeded a thousand.

At the center, surrounded by twelve colossal vessels of varying shapes and colors—some shark-like, others resembling mechanical jellyfish—was a comparatively modest ship.

The Twelve Siren Arbiters flanked it protectively.

Boom—

Once the entire fleet had assembled, it slowly advanced, drawing closer to the port.

Blotting out the sun, the Siren armada exuded a heavy, oppressive aura.

”W-What... that many?!”

The shipgirls and lolis on the beach stared, dumbfounded.

Tashkent didn’t even notice her ice cream falling to the sand.

”Hehehe—”

Aboard one of the leading ships, a tall, silver-haired, red-eyed woman dressed in silken veils rose from her seat and stepped out onto the command deck.

Empress surveyed the port below and nodded in satisfaction.

”Ah, such lovely reactions! See? I told you they’d be scared silly, but you didn’t believe me.”

”...”

Temperance and Lavos exchanged silent looks.

Empress opened communications to the surface.

”Children, we are the Siren Arbiters—representatives of the highest decision-making body of the Sirens, and the strongest force of the organization.”

Her pale, beautiful face curved into a faintly mad smile.

”Do not be afraid. Though the Sirens have fought against you for many years, we come without hostility...”

The Arbiter thought she had frightened the shipgirls and lolis below, so she tried to calm them.

Thinking about it, it made sense.

After all, most of them had only ever fought against the mass-produced Sirens—the highest level they’d seen were the Experimentals, and never anyone higher.

”Hm? Observer and Purifier are there too? What’s their reaction?”

Empress noticed the two Siren girls not far away.

”...”

Observer, seeing the massive Arbiter fleet descending from the sky, immediately sat up—and smacked Purifier across the face with a tentacle.

”Get up! Get ready to receive guests!”

”Our backwater country cousins have come begging the Starsea Empire for food!!!”

”???”

...

”...”

In the Azure Port, the vacationing shipgirls, lolis, and Heroic Spirits stirred briefly, gazing curiously at the mysterious fleet in the sky.

They instinctively assumed that some powerful enemy had breached the dimensional barrier and descended upon the port.

However, once they confirmed the newcomers’ identities, their tension quickly dissipated.

”Oh, so these are the Arbiters that Commander and Purifier mentioned...”

Formidable and Illustrious observed the descending ships for a moment.

In earlier years, such a sight—an overwhelming Siren fleet blotting out the sky—would’ve sent the port into full-scale panic.

But now...

”Wow, what a strange design! Though I imagine sitting inside one of those ships must be rather uncomfortable, poi~?”

”Who in the world designs ships to look like sharks and squids?! Wouldn’t that make installing new weapon systems a nightmare?”

”They look kinda weak. I bet they couldn’t even withstand a light spear or a few photoid bursts...”

A group of shipgirls stood below, casually critiquing the Arbiters’ fleet.

The Siren vessels certainly had a distinctive aesthetic—their hulls painted in shades of dark blue and black, adorned with marine-animal-inspired designs that were equal parts bizarre and fascinating.

”Yaaawn~ So annoying... can’t they have a bit of public decency? Bringing warships to a resort planet—what are they even thinking?”

”Did Observer not remind them this is a demilitarized zone?”

Indomitable lay lazily on a beach chair, hugging her pillow as she turned over.

She soon drifted back toward sleep.

”Mm... all Commander’s fault. Didn’t get much sleep last night... Zzzz...”

”...”

”Well, in any case, since they’re relatives of Observer and Purifier, we should still welcome them properly.”

Illustrious clapped her hands.

She gestured for the Bulins and Angeloids to make some room.

”Formidable, Victorious, Indomitable—come with me.”

...

Empress and the Siren Arbiters couldn’t help but find the shipgirls’ reaction a little odd.

They had been fully prepared for a fierce battle born of misunderstanding.

Instead, their supposed enemies merely looked curious—and surprisingly calm.

A quick scan revealed that the port now housed many unfamiliar girls.

Not only shipgirls, but also research lolis on vacation, Heroic Spirits, Spirits, and even wandering witches.

Some were sunbathing, others grilling skewers—all exuding an atmosphere of peace and harmony.

”So this is where the Experimentals live? Hmph, more like a paradise than a military base.”

A black-haired Arbiter snorted softly.

As the Sirens’ most militant division, the Arbiters were used to war—blood, steel, and endless battles.

To see such tranquil living conditions here felt almost absurd.

”Experimentals, report in.”

Empress opened a communication link to Purifier and Observer.

”Report this world’s situation and your current research progress.”

As the superior unit to the Experimentals, the Arbiters normally held an authoritative position over them.

No sooner had her words fallen—

”Coming—!”

Vmmm—

A surge of spatial vibration rippled through the air.

In the distant sky, six massive starships materialized in projection.

At the forefront were two colossal Siren battleships painted in black and crimson.

Their hulls were dozens of times larger than the Arbiters’ own, forged from dark alloy and bristling with menace.

While they retained the distinct Siren design language, their prows bore a massive axial cannon never before seen.

As for the weapon mounts along their flanks—there were simply too many to count.

Farther back were four Illustrious-class armored carriers, nearly identical in structure but differentiated by hull markings and color patterns.

Whether the enormous Siren warships or the Illustrious-class carriers, all of them utterly dwarfed the Arbiters’ entire fleet.

These were interstellar warships—built for combat across the void, not atmospheric warfare. The difference in size alone was staggering.

Compared to them, the Arbiter ships looked like mere toys.

The six starships hovered across from the Arbiter fleet—though few in number, their presence alone completely dominated the skies.

”W-What is that?!”

Confronted with a fleet so far beyond imagination, the Arbiters froze in shock.

Their instincts immediately pushed them into combat readiness.

”Hey hey hey~~!”

”Long time no see—welcome to the Starsea Empire!!”

Only when Observer’s cheerful voice came through the holographic channel did everyone finally exhale in relief.

The little loli stood outside her command bridge, waving energetically toward the Arbiters in greeting.

At the same time, another voice joined the channel.

”Good day, everyone.”

”This is the Starsea Empire’s First Subordinate Universe—Azure Lane Port District.”

”I’ve heard about you from Commander, as well as from Observer and Purifier. It’s truly an honor to finally meet you.”

Illustrious spoke with her usual grace and poise, her tone carrying that unmistakable royal elegance.

”I am Illustrious, first ship of the Royal Illustrious-class aircraft carriers. The Commander is currently occupied with important matters, so I have come to greet you in his stead.”

”...Huh?”

The Arbiters were momentarily stunned.

They stared at the massive starships dominating the sky—utterly unable to reconcile them with the fragile, petite figures they once knew as shipgirls or Experimentals.

Illustrious-class aircraft carrier... Number One?

Empress vaguely recalled the name and immediately pulled up the database.

[Illustrious-class Aircraft Carrier No.1 – Illustrious]

A core member of the Royal Fleet.

Hull length: 230 meters.

Full-load displacement: 29,620 tons.

Maximum speed: 30 knots.

Waterline armor: 114mm. Bulkhead armor: 64mm.

Equipped with 36 carrier aircraft, 14 anti-air guns, and one radar system.

Specialized in mid- to long-range support operations. Generally fights in humanoid form; occasionally manifests her full hull.

Recorded 126 engagements against the Experimentals, destroyed XXX mass-produced ships, fought the Omitter five times with 3 wins and 2 losses...

The data—originally uploaded by Observer—was detailed and precise.

Empress read through it line by line.

Then she looked up again at the colossal, pure-white starship floating in the sky, easily tens of kilometers long.

Her expression said it all: You’ve got to be kidding me.

”This... THIS is Illustrious?!”

”You’re telling me that massive ship up there—flying through the sky like a moon—is Illustrious?! Don’t tell me the ones beside her are Formidable, Victorious, and Indomitable?!”

”It’s me,” Illustrious replied calmly. ”I assumed Observer had already informed you of our situation?”

”...That girl’s been uploading nonsense to the database lately—either birth logs for new shipgirls or weird diaries. Who knows what’s even going on anymore.”

During their time stranded in a parallel universe, their knowledge of the Starsea Empire came only from the fragments Observer occasionally uploaded.

Unfortunately, after joining the Empire, Observer had completely let herself go. Instead of proper reports, her uploads consisted of things like, ’Helped deliver three new shipgirls today,’ or ’Had fun flirting with Commander again—so tired but so happy~’

No wonder the Arbiters were utterly confused.

Their gazes turned toward Observer and Purifier’s ships.

They, too, were vastly larger than before.

Originally, the Experimentals were Siren researchers—known for their intellect rather than combat power. In the past, they had always treated the Arbiters with awe and respect.

But now... looking at Observer and Purifier’s newly refitted forms, the Arbiters felt a rare sense of intimidation.

Stronger than them.

That was the collective, instantaneous conclusion of all twelve Arbiters.

They couldn’t even identify the weapons mounted on those hulls.

Photon projectors, tachyon lances, erosion missiles—technologies gathered from countless worlds—completely outside the Arbiters’ understanding.

Yet by instinct alone, they knew those weapons were capable of apocalyptic destruction.

”Your hulls... were they modified by Compiler?” Temperance asked cautiously.

”Nope—by the Commander.”

Observer immediately puffed up with pride and cleared her throat.

”The Commander led us through multiple worlds, integrating various advanced technologies into starship-grade refits! Ah, those little lolis and Bulins are really amazing, you know~”

”Pretty impressive, right? Anyway, now that you’re in the Starsea Empire, as long as you follow the Commander’s orders, we’ll make sure you’re well taken care of.”

”This is the Empire’s resort sector, so please don’t scare the others. Stow your ships and come land with us.”

Purifier was still busy relaying messages to the shipgirls below:

”Everyone, don’t panic! They’re Sirens too—used to be our superiors!”

”Uh... the Arbiters might be a bit out of touch after staying in foreign worlds too long. They’re just not used to how things work here in the Empire, so... try to be patient with them, okay?”

”???”

The words sounded polite—almost overly polite.

But to Empress and the other Arbiters, something felt horribly off.

What did she mean ’we’ll take good care of you’?!

She was clearly speaking from the Starsea Empire’s perspective—as if she herself were part of it, addressing the Arbiters as outsiders!

Had she already stopped identifying as a Siren of the Experimentals... and now seen herself as a shipgirl of the Starsea Empire?!

And then she had the gall to say the Arbiters were ”uncultured country bumpkins” who needed to be treated gently?!

So now the Experimentals were treating their own superiors like primitives from a backwater tribe?!

—A complete inversion of heaven and earth!

—

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