Author's note: Written for the Writers Anonymous All/No Dialogue Challenge as such, this story specifically contains no dialogue at all. It contains my theories of how I think the anime will go and why things are the way they are.
The Unending Cycle
Humanity had entered its golden age. They discovered the key to immortality, all it took was harnessing the secrets of magma. They devised miraculous machines to keep them alive indefinitely. This continued on for several generations before humans became selfish where prolonging their own lives were more important than creating new ones.
The earth itself was drained of resources by the undying population. They trespassed onto the habitat of ancient creatures: the klaxosaurs. Magma was sacred to them and humans were using it to prolong their lives and in other unnatural ways. Their existence was an anomaly because they could only be harmed by their kin or blood. The first klaxosaur leaped out of the volcano invulnerable to everything humans could launch. It easily destroyed multiple cities regardless of the valiant human resistance.
The humans were persistent in fighting the beast despite no chance of success. They happened to discover it's weakness, its own blood that it sometimes shot as projectiles out of its body. The beast was harmed but quickly regenerated by its own metabolism. They were immortal except for destroying the core that can only be damaged by its own kin.
Humanity experimented on the blood as their cities crumbled into ruin under the beast's relentless assault. Humanity took unwilling volunteers and injected them with the beast's blood. To their horror, they created more klaxosaurs as the humans became monsters but now they had more blood to use. They kept these participants caged for an endless supply and experimenting upon them with various weaponry that also proved ineffective.
A madman by the name of Dr. Franxx decided that it would be a good idea to infect children with the blood. Humanity had given up procreation long ago, thus all the children were bred for a singular purpose: to defeat the klaxosaurs. They weren't kin, they were expendable against an indomitable foe.
Children were carefully observed but they too turned into klaxosaurs when reaching adulthood. All of the klaxosaurs must be caged because they cannot be destroyed. Through poor experiment planning, the staff witnessed children kill each other. They freely tried to kill children themselves only for them to become beasts even sooner. The good doctor took notes and decided the children would be trained for combat.
The children were still human even though they were destined to become beasts later on. They were given a classification: parasites. They were living off of humanity's goodwill, to eradicate a problem. A singular purpose was their only goal. Anyone who didn't fit the standards must be culled. The failures would also be good target practice for their kin. It was alarming that they had to be culled before a certain time. The Klaxosaurs would grow too powerful for normal containment measures otherwise. The beasts utterly loathed humanity, thus anyone of low potential could be discarded that way but it seemed like a waste of sources.
It was time, the first wave of supremely well-equipped children were ready to fight. The beasts decimated the children effortlessly although it suffered some permanent wounds. The good doctor took even more notes, the ones who were successful, displayed some form of affection towards each other.
A machine heralded a new era for humanity, the Franxx. The Franxx ran on the power of emotion and also utilized the pilots' blood as a power source. It was a way to create a mockery of the first klaxosaur into tricking it to believe that they were kin. There were no stronger emotions than love. Male and female pairs were trained to pilot a robot together. The children were further given classifications, stamens, and pistils. They were to be remembered as objects and tools, nothing more. The floral classification served two purposes, it helped the children bond as well as help the creators remember that they don't matter.
As humanity experimented with the blood and their offspring more and more, they were fortunate enough to have specimens that were mostly klaxosaurs but still sane enough to regard the rest of humanity. A klaxosaur capable of emitting human emotions, they tested the weaponry and recorded the cries, the rate of healing for future use. It was carefully studied in the Garden, the first underground city.
Depending on the infant, when they were infected, they displayed different symptoms. Some became beasts immediately, while others took their time but becoming a beast before adulthood was a certainty.
Humanity celebrated at the successful creation of subject 002. It was the test subject to display human emotion while being fully klaxosaur like from birth. It was a very promising prospect. She had red skin and blue blood. They had special plans for her, she would be tortured yet manipulated into caring for one person alone. Deviation from the plans was not allowed.
A disaster struck their plans. One of the other test subjects found subject Zero-two. He displayed compassion towards her. Nothing was allowed to awry. They decided to erase their memories after some debate, the experiment would continue unabated. She had to trust the one human they selected for her instead of another parasite that was destined to become more monstrous.
Due to her special design, anyone that was her fighting partner died after piloting a Franxx for three times or less. Her conditioning made her more willing to kill more of her kin. It was also a way to discard low potential parasites. They would be brainwashed, willing to ride with Zero-two until they died and surely they will. She absorbed the klaxosaur blood from them and one cannot survive without it after they've been infected.
Zero-two travelled with her commander whenever she was needed most. They conditioned her well enough to mercilessly kill her kin with the vain hope she could become normal to be just like them. It was never going to happen, it never will but it was fine as long as she believed. Her function was to obliterate her kin, as well as recycling, failed parasites. Parasites could not die unless if killed by one of their own.
No one at the high command cared or remembered when Zero-two came into contact with the meddling parasite from years before. The took new interest in Hiro due to his bravado in trying to pilot a Franxx with her and succeeding despite failing at piloting a Franxx with his assigned partner before or anyone else. He was even able to operate the machine without being forcibly reprogrammed into an obedient slave. They decided to let this continue because they found it interesting.
The high command was pleased when Hiro and Zero-two displayed ruthless efficiency at killing Klaxosaurs. They were running short on failed test subjects anyway, thus they allowed this to continue. Hiro was classified as a failure thus if he died because of her, it was no loss on their part at all. It was just amusing that he didn't have to be reprogrammed.
Hiro surpassed their expectations by surviving piloting the third time with her. By their calculations, Hiro would have died. They took a special interest in the pair. Their bodies must be studied to identify any anomalies and in the case of Zero-two, studied even more.
By all the odds, Hiro survived a fourth time somehow. This made Hiro even more important to monitor. To their surprise, instead of draining his klaxosaur infection, like Zero-two did with countless others. Hiro's infection somehow becomes more pronounced.
After yet another flight, Hiro collapsed finally from the strain of piloting with Zero-two instead of dying, he was going to become a klaxosaur way before the appointed time. His plantation seemed to care about him, they observed. They decided that Hiro was too important for him to pilot with Zero-two again.
They ordered her away back to her usual task of killing failed parasites as well as eradicating her kin. They were curious if Hiro could serve to dispose of failed female parasites the same way Zero-two did by draining the klaxosaur infection from them through fighting in a Franxx.
The high command was pleased when his own plantation decided to try to pair him with another female. They were absolutely delighted when Ichigo, one of the parasites he knew from childhood died through the strain of operating a Franxx. It was a resounding success. They finally had a way to dispose of the females before they turned. They finally had a male klaxosaur that could understand human reasoning, just like Zero-two was capable of. Their very existence could eradicate all presence of parasites and klaxosaurs alike.
It went better than expected when in an attempt to cheer up their friend, more parasites perished. It conserved resources when parasites die without turning while killing others in the process. Hiro wanted to remain alive to see Zero-two once again. They used that to keep him fighting. They both wanted to see each other, something they would never be allowed to once again. It ran in contradiction to their plans of eradicating every klaxosaur in existence. The monsters must be destroyed so humanity can prosper once more.
The war against the klaxosaurs raged on until there were just those two left. They killed every kin, used up every failed parasite. They were finally allowed to meet. The high command hoped that by meeting each other, they'd find purpose and exist peacefully with humanity. klaxosaurs were immortal after all and can only be destroyed by themselves while using specifically crafted weapons.
Zero-two and Hiro told each other of the many tribulations they faced. The partners they've seen dying in accomplishing their given tasks. They decided to live out their remaining lives peacefully ever fearful of becoming monsters themselves. They were done fighting.
Humanity was saved and the monsters were defeated.
Author's note: Constructive criticism appreciated.