So this is the result of a random conversation between me and my brother after watching Ultraman Nexus, and playing the english translation of Sakura Wars (alongside the anime, OAV and manga). There are some key points we saw that reached this conclusion:

- Kouma (generally called Demon in Sakura Wars) and Space Beasts function relatively the same, being born and empowered by human's negative emotions, and Kouma only lacks the explicit nature of devouring humans on-screen but it's not too far-fetched to assume that they also consume organic creatures, preferably humans.

- Ultraman Nexus' design greatly leans toward Japanese culture with the head being based on a kabuto helmet and his silver body being a hakama but simplified and skin-tight.

- Nexus' host can greatly strengthen Ultraman and also weaken him, similar to how Spiritual Power works.

- Nexus' name also ties to its message: the light is also the bond. And the main cast of Sakura Wars always forms a strong bond that helps them overcome many struggles.

Obviously, these details don't mean Ultraman Nexus can flow well with Sakura Wars so think this work as a semi-serious experiment. As such, there are significant differences that must be made:

- Space Beasts won't appear much or even at all. Kouma, however, becomes much more ferocious and powerful, but not so much that the main cast cannot defeat without Ultraman's help.

- At the time of writing, we haven't thought of introducing multiple hosts and the story only has one non-Sakura Wars protagonist as Ultraman's host.

- Both of us are not good at writing romances and seeing that this story uses the game (original and PS2 remake) as main inspiration, Ogami, Shinjiro and the protagonist all have their own love interests that later form a harem for each, mostly for comedic purposes so please treat this aspect lightly and not overly serious.

- By default, Ultraman is very huge and tall. He may shrink down to fight alongside the Koubu, Sakura Wars' mecha.

- The Light can be empowered by Spiritual Power so for example, if the protagonist gets injured and treated by Iris, he will recover faster and return to fight as Ultraman. However, this doesn't mean anyone can become Ultraman without being chosen by Noa's will. Exceptions may happen but they are not guaranteed.

- Dark Zagi doesn't exist but Dark Ultras do and their purpose is to bring him to this universe.

Alright, time to end this lengthy foreword. Let's dive into the story, shall we?

Humanity had been at war with supernatural beings ever since two sides first contacted each other. Whether it would be vampires, zombies, ancient demons, humankind would always prevail. This brought us to Japan and its struggle against a demonic race born from vengeful spirits of the deceased, known as Kouma.

Throughout the years, with the discovery of humans with latent Spiritual Energy that formed the Anti-Demon Squad, Kouma was thought to have been driven to extinction. However, in Taishou 7 (1918), there was the largest Kouma incursion ever recorded. Conventional weapons could only do so much against the demons, and they drove mankind to near destruction.

At the darkest moments, when Yoneda and his teammates were exhausted and powerless against the gigantic demons in Aoyama, a miracle happened.

"Do not give up!"

"Eh?"

Ayame Fujieda heard a voice as the demon's claws brought down upon her. A stream of red lights came from the sky and obliterated the Kouma. From the spot where the light was, three arc-shaped projectiles shot out and knocked back the other giant demons that were cornering Ikki Yoneda, Shinnosuke Yamazaki and Kazuma Shinguji.

"What in the world is that?"

"A silver…giant?"

As Yoneda said, a towering giant in silver rose from the crater and stood against the demons. One spilled out multiple fireballs that barely left a mark on the giant's body and another used its claws to attack him.

"DEYAA!"

The giant used his braces to deflect the claws and slash them off, causing the demon to screech in pain. He then crossed his arms to gather energy and released a larger version of the earlier projectile, slicing the Kouma into two clean halves. The victory was short-lived when one demon managed to sneak past the giant and grab him from the behind.

"GUAAAAAH?!"

Its brethren used this opportunity to lash out at the giant, clawing and spitting acid fireballs. The giant grunted in pain, snapping the Anti-Demon squad out of their trance. Yoneda nodded at his subordinates to coordinate an attack, with Kazuma unleashing a devastating attack called Ouka Houshin that was strong enough to blow the holding Kouma away, freeing the giant. Yoneda and Shinnosuke crippled its legs and Ayame delivered the killing blow on its head.

"Do it!"

Yoneda shouted at the giant, who nodded and took a stance similar to drawing a sword, lightning sparking between his palms. The giant then formed a cross with his hands, releasing a beam of light hitting the Kouma and reducing it to particles with no chance of regenerating. The military men and the squad burst into cheers at the total victory. The giant gave them a thumb-up before disappearing in a light pillar, much to the confusion of those who were there.

Three months later, an even bigger Kouma manifested in Nihonbashi and led an army of smaller demons. They pillaged and devoured any humans in their sights, causing a massive uproar and despair to Japan. Eventually, the Anti-Demon squad arrived. Yoneda and his teammates held the gigantic Kouma long enough for the Army to deploy large cannons to bombard the area with heavy artillery. Still, that wasn't enough to defeat the horde of Kouma.

"At this rate, I must use it!"

Kazuma hurriedly left the medical tent with a wooden box. Yoneda saw this and went to stop him, cursing the latter to drop the box and reveal a set of three artifacts: the Majinki.

"No! I will not allow any of you to sacrifice yourself!"

"But sir, at this rate, the capital will be destroyed!"

"No means no! Kazuma, you go back and tend to your wounds. This is an order!"

As Yoneda made his decision known, the ground shook violently as the result of another gigantic Kouma. The old general stared hopelessly at the two rampaging monstrosities.

"Is this…the end of mankind?"

"...No." A voice belonged to a young recruit of their team. A young officer with adequate spiritual power named Shin. "Humanity will not lose to beings of darkness like Kouma. As long as there is Light within your heart, a miracle will happen."

"Shin-kun…"

Ayame whispered behind him. Shin turned around and smiled at her, taking out a white object from his jacket.

"The bond of people creates Light, and Light brings about Miracle. That is, Nexus."

"Nexus…"

Yoneda muttered, wondering what it truly meant. Shin bowed at the general in gratitude and glanced at the gargantuan demons.

"Thank you for teaching me about this world, Yoneda-san. Ayame-san, I greatly valued our time together, despite how short it was. Kazuma-san, send your daughter my regards. Shinnosuke-san, I hope your dream will come true."

The white object, Evoltruster, pulsed like a beating heart as if to urge Shin to do something. Shin smiled warmly at the humans before removing the cover on the device similar to how a samurai drew his sword from the scabbard just as the Kouma attempted to trample him. An explosive burst of light erupted Shin and formed a light pillar that repelled the demon. At this moment, Yoneda finally knew the real identity of Shin Katsuragi: he was the silver giant that had saved them in Aoyama.

"SHUUUU…"

Shin, no, Nexustook a fighting stance at the surprised demons. The giant delivered a glowing punch at one of the Kouma and grabbed it, separating the two. He looked back at Yoneda, as if to tell him to handle the other. Yoneda found his resolve and nodded back.

"From now on, the Anti-Demon squad will proceed to exterminate the demon! While Shin handles the flying type, we deal with the ground one."

"Yessir!"

The battle of Teito (Imperial Capital) concluded with the sacrifices of the silver giant Nexus, who took the gigantic Kouma duo with him in a bright light, and Kazuma Shinguji, who performed a ritual to seal away the remaining demons at the cost of his own life. The Army still wished to replicate the miracle that was the silver giant. Numerous inhumane experiments were conducted based on the testimony of those who were present when Shin Katsuragi became the Light, transforming into the giant. Eventually, when the spirit armor Oobu was introduced, there was no male soldier with sufficient spiritual power left to operate it, leading to the Army begrudgingly allowing young girls to be pilots of the Oobu (and later, Koubu).

People of Japan entered an uneasy era of peace, with few occasions of Kouma escaping the seal that were dealt with by a new combat organization called Teikoku Kagekidan (Imperial Assault Troop, Teigeki for short), the Anti-Demon squad's successor. Thanks to the Koubu, the secrecy of the organization was maintained well. However, there soon came the time that the Teigeki needed a capable commander to lead them.

Taishou 12 (1923), a young man in black clothes groggily woke up from an alley, the only belonging he had was a darkened wand with a crystal in the middle that kept glowing like a beating heart.

"The Light… I can't feel it anymore…" The man said to himself, repeating the sentence like a mantra. He dragged himself weakly out of the dark alley and wandered aimlessly on the streets, drawing the attention of bystanders. He kept walking until there were screams and a bestial roar ahead of him.

"Beast? No…" The young man tried to focus his eyes on the winged, purple monster being attacked by the police. The monster seemed to notice only him and made a beeline toward him.

"What is he doing?! Run, citizen!"

"I… I have to… fight." The wand's light pulses became more urgent as his eyes and mind became clearer. "Kouma…? Yes, I'm here…" The man held the wand with both hands on two ends. "To defeat Kouma! WOOOOOAH!"

He cried out as he pulled the wand open, revealing the complete crystal. Light energy from the surroundings converged into his being and the wand before exploding and forming a pillar of violet light. Everyone, even the Kouma, shielded their eyes until the light died down.

It was a giant of light, as an old police officer exclaimed. He might look different than the silver giant that saved Teito five years prior, but the policeman was confident that the giant was here to do the same.

The Kouma screeched in anger and spread its wings to attack the giant, but he slammed his fist right at the demon, knocking it down mid-flight.

"DERYAH!"

The giant touched one of the bracers on his arms and fired a dark energy blast at the Kouma, blasting its abdomen. The monster spat out acids in retaliation and hit its mark as the giant grunted in pain. Seeing this, the police rallied themselves to find a good cover and resume firing their guns at the demon, preventing it from releasing more foul breaths.

"Hm!"

The giant nodded at them in gratitude and began to charge up energy into his right hand. The moment the Kouma realized what he was doing, it was too late. The giant unleashed a powerful burst of energy entering the demon from its jaws then exploding from inside. The Kouma stood still for a moment and could only whimper before exploding into light particles. The policemen and citizens cheered at the giant's victory.

"YOU SAW SOMETHING YOU SHOULDN'T HAVE, HUMANS."

An echoing, deep voice spoke up from the giant, surprising everyone else. A green light enveloped the area as the humans collapsed. After waiting for a while to make sure no one was awake except a suspicious balloon airship on the sky, the giant crossed his arms and vanished from the sight. Onboard the airship, two military man and woman watched the scene in silence. One of the operators reported that there were no energy readings in the area, confirming that the giant was truly gone.

"Commander, was that Shin?"

"I can't say for sure, Ayame-kun." The man, Yoneda, tugged his hat and grimaced. "I can sense a very deep darkness hidden within that light. Almost like two sides are fighting for dominance. I have a bad feeling about this."