During the age of Imperial Japan there was a young Zainichi Korean couple in tremendous debt. When they found out they would be having a child, it wasn't even a question of whether or not they were keeping it as their immediate thought was they didn't have the money to support a child. When the woman had given birth to a baby girl, the family left her on the doorstep of a Japanese couple who seemed to be better off than the Zainichi Korean couple.
The young girl grew up with what she thought were her biological parents. Mister and Miss Saiki were certainly not going to tell the truth to their quote on quote daughter whom they named Aiko. She grew up as normal as one could in Imperial Japan, in minimal poverty and certainly not struggling as much as other families in their neighbourhood. However Aiko's normal life turned upside down when she was only seventeen years old. She had encountered a man late at night and although he seemed a tad bit older than her, his personality didn't show it. It wasn't all that long until the pair struck up a conversation.
How could a normal conversation turn Aiko's normal life upside down you might be asking. Well it wasn't until the man, who later turned out to be the river deity Phlegethon, forced himself upon her. She kept it a secret, but when she had gotten pregnant there was no way to keep it a secret. It was at this time that Aiko thought it'd be best to leave her parents behind and what better time as there was an island with a newly formed kingdom. She was unaware of what the king was like, but that wasn't going to stop her. She left the night after finding out she was pregnant.
Eventually arriving at the island kingdom, Aiko worked her way into the kingdom by becoming a servant to the king Sakiyama Osamu. This island she had ran off to was deemed Orankoshi, and it seemed to be the best place for her to start a life of her own. It was a hard decision on whether or not she would be keeping the child, but by the time she found out she figured she was already too far along to even have a choice. Her son was born four months after her arrival to Orankoshi on August 18th of 1902 and he was deemed Saiki Kaito. The baby boy was two years younger than the prince of Orankoshi, Sakiyama Fuyuki, someone who the newborn would become quite familiar with.
Kaito grew up inside the guarded kingdom of Orankoshi. He lived a somewhat structured life, he would do his best to help his mother in tending to the king and prince, and in his spare time he would try to interact with the other children in the kingdom. It was an odd way to live, tending to someone only two years older than him. But if he didn't there would be severe consiquences for both himself and his mother. At the age of ten years old, ancestors of Aiko's had found her and informed her that while Japan had Korea under their reign, her biological parents were killed. At first Aiko didn't know how to feel, after all the only parents she had ever known were the Saiki's. But once she had fully processed it all it became to much for her and she took her own life a few weeks after.
Now considered an orphan, Kaito was taken in by a noble couple who were both infertile and had given up on trying to have a child of their own. Despite being taken in by an incredibly loving couple, Kaito was never truly the same after his mother's death and thus he vowed to someway get his revenge on anyone he could. Was this logic flawed? Most definitely as he had no clue who had executed his long lost grandparents and indirectly caused his mother's death. But if he could get his revenge on anyone, he figured it might as well be someone in a high up position.
After the whole accident, Kaito kept a low profile. He stopped serving the king and prince as much, partly because of his new parents, and he also took up sword-training for future purposes. The first time these skills came into play was when Kaito had turned thirteen and he had encountered his first monster. For whatever reason he had wandered outside of the kingdom's walls in order to get some live practice on animals and such. But he bit off more than he could chew when he was attacked by a hellhound. The inexperienced demigod was unable to fend the beast off, but luckily a guard had managed to distract the hellhound with arrows to the back long enough for the young demigod to get back inside the walls.
It was safe to say after being attacked by the hellhound that Kaito never wanted to step foot outside of the kingdom again. All he would do was stay inside the walls, train with his sword, and integrate himself back into the kingdom. It wasn't until he was sixteen that the current king Osamu had been overthrown by Fuyuki, a usurping in which Kaito fully supported for a plethora of reasons. He figured it would have been easier to get revenge on someone closer to his own age. But this plan later shattered into as many pieces as possible when Kaito and Fuyuki slept together. It was only one time, and Kaito knew Fuyuki was nothing more than what we nowadays would call a f*ck boy. But he couldn't help but fall for the king.
Unfortunately Khione was not impressed by her son Fuyuki during his time as king and thus she brought on many famines, one of the last few famines Kaito perished in at the age of eighteen. Once arriving to the underworld, instead of going to Hades like most, Kaito's father intervened and took him to his own domain, formerly introducing himself as Phlegethon at this time as well. After bringing his son back as a fire spirit, Kaito was stuck with his dad in the underworld, a place he didn't enjoy at all if he was being completely honest. But alas he was stuck their for a century. Over his stay he grew to enjoy it one could say, after all he was sexually involved at times with Persephone. But when 2016 had rolled around it was time for his return to the over world. Shortening his name to a more modern version, aka Kai, he was sent to Camp Half-Blood by Persephone and Phlegethon.
At first Kai was hesitant to go to Camp Half-Blood. He really only understood basic English and felt far more comfortable in speaking Japanese and Greek. Picking up on Kai's worries, Persephone requested the help of Mnemosyne to give him full understanding of the English language, leaving him with little to no accent at all, as long as he didn't get too ahead of himself and start speaking too fast for his brain to keep up that is.