The backstory of this silly little fish guy begins long ago, when a meteorite hit the Earth
also i uh. ended up accidentally rambling for 1k words. if you don't wanna read the whole thing here is a tldr!!
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the meteor had crashed upon the Earth and brought with it terrible, terrible things, and had split open the Earth, letting the sinful things from those down below begin to infect the people. Thanks to it, a terrible illness—a Curse—began to plague the people. In their despair, they sacrificed those who were unaffected to the Ocean. But it didn't work. Nothing worked, until they tried something cruel—eating the sacrifices. They were stripped of personhood, killed, and consumed by those who were sick. Thus, the illness was cured. In fear that the illness would come back one day, the relatives of that sacrifice were treated as Gods.
At some point, the first sacrifice had returned to the land from the Ocean, and with one of those Gods they started a family. That family is Kanata's lineage.
This family—the Shinkai (meaning deep sea) family—were incredibly powerful in their area, and they began to raise their son Kanata as a God as well. Kanata was locked away in his room, surrounded by nothing but water, and told he was a God who would grant wishes for those around him. He was isolated from the world, incredibly lonely, and only existed to be God.
During his early childhood, a family who participated in this cult—the Mikejima family—had frequently visited. Alongside Mr. Mikejima was his son, Madara, who was the same age as Kanata. Madara himself was a lonely boy, so when he saw the suffering Kanata, he wanted none other than to be that kid's friend. As often as he could, he would hang out with Kanata, telling him things about the outside world, bringing in hidden toys and magazines, playing games with him, an assortment of things. Though Kanata would say harsh things to Madara, he didn't mean them; really, he saw Madara as his first hero and his first friend.
The defining moment in both of their childhoods is the moment when Madara made his first and only wish to Kanata: to cure his younger sister of her terminal illness. The problem was, though, Kanata was never really God. Most wishes he granted were fulfilled by his followers. But he had no way to cure Madara's little sister. Kanata initially thought that he just wasn't good enough, but Madara had realized the truth that he'd been suspecting for a long time. That Kanata was not a God at all, and was just a human boy. From this point on, Madara had made it his life's mission to save Kanata from this cult, and allow him to live freely as a human.
When Kanata is finally old enough to attend high school, he ends up going to the idol school Yumenosaki Academy, alongside his friend Madara. See, many of the magazines that Madara had brought for Kanata were about idols, since Madara himself wanted to be one. Kanata ended up staying a solo performer, but his elegance and notoriety around the area for his Godhood gave him some influence in the school. Madara was well off too: he became the vice captain of the long-time standing unit RYUSEITAI. Also during this year, Kanata meets another peculiar RYUSEITAI member, Chiaki Morisawa. Chiaki perplexes Kanata because he is the only person who has not asked Kanata for a wish: the only one who truly treated Kanata like a human-being.
The boys' first year was decent enough, but their second year is shaken by the notorious enstars event known as "The War." Because the school was in such terrible condition, one Eichi Tenshouin, alongside his own childhood best friend Keito Hasumi, and a unit he crafted known as fine, had started a revolution to establish control within the school and fix it. The problem is, Eichi needed a scapegoat for this revolution so he could gain support. What better group to blame then the supergeniuses of the school—five individuals he would nickname the "5 Eccentrics/Oddballs," donning them a title that mocks their bizarre circumstances and personalities. One of those strange individuals happens to be Kanata Shinkai. The Oddballs form their own weird but incredibly tight-knit friendship due to the fact that they are the only people who can understand each other.
In the war, Kanata is the second of the 5 Oddballs to be "executed"—rather, humiliated and stripped of their power within the school, and becoming a victim of the student body's hatred. Keito, who was raised Buddhist, has no connection to Kanata's religion and makes it his goal to disprove Kanata's state of Godhood within the school. Too many students began to just wish for Kanata to do things for them, which he granted every time, breeding a new sense of laziness in the school. Keito took advantage of Kanata's inability to deny requests, wishing that Kanata would tell everyone that it is not he who is granting the wishes, but rather the student council that Keito and Eichi belong to. This way, Kanata is painted as a fraud, and Keito gets to take all the credit for the good.
After this, Kanata's reputation is ruined, he becomes the victim of bullying, and he even attempts to drown himself. Keito had challenged Kanata to a competition that he calls "The Battle of the Sea God," where his unit of AKATSUKI would compete against Kanata on a live stage and he would finally execute Kanata once and for all. Since Kanata was not registered to a unit, he was able to have Madara, who was still the vice captain, quickly admit him to RYUSEITAI while Madara was off in Brazil, and Chiaki performed with Kanata up on the stage. Keito still declares himself the winner over some stupid loophole he found, but nonetheless, Kanata's execution had taken place, though he now had a unit to call his own and was finally able to live as a human thanks to Chiaki being by his side when he needed it most. Starting his third year, Madara would abandon his position of vice captain of RYUSEITAI and give it to Kanata, while Chiaki finally becomes the captain he had always wanted to be.
And if you're curious where Madara was for the majority of their second year, the answer is actually quite bitter. Madara had continued trying to save Kanata from his cult, but the Shinkai family had caught word of it, instructing Kanata to keep his distance from the rebel Madara, and attempting to send Madara away from their son. It doesn't help that Madara is trying to help one of the 5 Oddballs, and Eichi sees him as a major threat, using his own wealth and resources to keep Madara away from Kanata and isolate the child God further.
tl;dr | There's a family line who were treated as Gods, which Kanata is the son of. He was isolated and raised as a God with only one friend. He went to an idol high school, was humiliated for his swag, and found humanity and new friends, and is now a silly fish guy.
While I'm here, I may as well also mention my favorite stories featuring Kanata! Just in case anyone would like to do further reading. I'll add more stories when I get the time :)
Purple Wisterias of May is an event story where Madara returns to Yumenosaki and recruits the track team and his childhood bestfriend Kanata, along with some of Kanata's friends, to perform at a festival with him.
Aquarium is a event story where the marine bio club, alongside some of Kanata's other friends, try to find him at his family's aquarium after he goes missing for some time.