He originally came along with all of the other Oni when they were looking for the FSM, and along with Mystake were among the only ones to stay in the realm survive. Kai was pretty young when he was first brought, only in his early teens and by the time canon rolls around he’s a young adult.
He ends up sort of living with Mystake, sometimes helping out in the shop and such, but he spends most of his time traveling ninjago for no real reason other than he can. During their search for the FSM he was never allowed to just. take in the scenery and enjoy the places.
But more often then not…..he’ll end up finding a child and, granted that they don’t have parents(or have bad ones), he’ll take them back to Mystake. Both of them have lived hundreds of lives, sometimes as a different gender, sometimes as a completely different species, and sometimes not even a sentient one(one of Kai’s favorite lifetimes had to be the one where he was a dog and just went around getting pets and playing with kids), but they always did their best to keep a bit of distance from Elemental Masters, especially Wu and Garmadon.
However, then things got….complicated.
It started with Kai visiting Ignacia early on in the life he was “Kai,” when around his early/mid teens and he found Nya. He ended up playing with her for a while before she asked if he could stay for dinner, which after some apprehension agrees as long as her parents say yes. Which they do, because Ray and Maya are just nice like that.
Fast forward a year. Kai makes semi-regular visits and likes to bring little gifts from his travels for Nya and has even brought Mystake over a few times.
However, this time was different. There was another man there, a man by the name Dr. Sanders Saunders. He was polite enough and had a bit of an accent, but nothing seemed overtly wrong about him.
What was wrong was with Ray and Maya. There were tense, when Saunder’s back was turned their eyes darted over to the doorway leading to the back where Nya loved to play. They were scared.
And Kai….honestly doesn’t know what he can do. He’s not sure if he should fight Saunders or not. He could be working alone or he could be part of an organization that will cause more problems. So he does the best he can and offers to take Nya to town and that he can take care of her. Both Ray and Maya understand what he means and agree, but not before Ray thanks him and asks if Kai could fix the sign since he’d getting too old to climb up the building(definitely a lie, the last time he visited Ray did a backflip off the roof because someone dared him to). He then pats him on the back and Kai could’ve sworn he felt some kind of spark, but later when he checked there was nothing on his shirt.
So yeah. He heads back to Mystake’s, drops Nya off and says he’ll be back soon. He then heads back to Ray and Maya’s place and finds them missing. He climbs the roof to check out the sign, at which he finds the map for the Golden Weapons, though he doesn’t know that. He waits three days before leaving and starting his search for what the map leads to and Dr.Saunders.
Imagine his surprise a month later when he finds the Sword of Fire.
Imagine his surprise when he picks it up and it actually works.
That’s when Kai realizes three things.
1) Ray was the Master of Fire
2) Kai is now the Master of Fire
3) Mystake is going to kill him
Fast forward a few years. Kai’s doing pretty well in being a self taught EM and his powers are coming along pretty well(as well as he can without unlocking his True Potential), his skill with swords is shining through(he takes the Sword of Fire with him almost everywhere), Nya’s growing up to be a fine young woman and helps out around the shop quite a bit, which is doing really well, and Kai was able to find the rest of the Golden Weapons and they are in a very safe place(he collected them because he thought that’s what Saunders/Krux was after, he didn’t know that it was something personal).
Also his fire is…..a lil weird. It’s usually purple and blue, sometimes even black, and he can choose when he wants it to burn something, so when he traveling alone he’ll have a little fireball over his shoulder to light the way(not that he needs it since Oni can see pretty well in the dark but still, the aesthetic). However when he does want it to burn there is nothing that will put it out until Kai makes it.
Meanwhile, Wu is not having the best of times. Ray and Maya are missing, the map was stolen, all the Golden Weapons are gone, and he still can’t find the Master of Fire because he knows they had a kid but has no idea where they are. At least the dragons came to the Monastery.
(Also, the dragons have really mixed feelings about Kai because on one talon Oni but on the other Elemental Master, so they mostly opt for just keeping him at a distance, however they won’t go out of their way to attack him.)
So he ends up opting to go to Mystake’s for some tea. He hasn’t been there in a few decades and could use some to relax.
What he doesn’t expect is for them to be fighting off some Skulkin.
This time it’s actually Mystake who gets snatched, as Nya was at school when it happened, which extremely pisses off Kai, enough that he almost swaps back to his Oni Form.
However what the Skulkin don’t expect is him whipping out the Sword of Fire and start shooting fireballs from his hands like a pro.
Wu’s pretty sure he just found the Fire EM.
But yeah, Wu then offers to help train Kai, Kai decides “what the hell I don’t have anything better to do” and agrees, but makes Wu wait until Nya comes home because he isn’t just going to leave her alone.
So she gets trained as well.
And for a while canon is pretty similar, only this time around Kai is more of a parental role to the ninja. Also this lad is absolutely Chaotic Good. Can and will stab someone.
When Kai first meets Garmadon he’s so tempted to reveal himself and chastise him like a parent because Garm is a few thousand years younger than him.
During their first meeting Lloyd is dealt with much better, especially since Kai has a lot of experience with kids…..but he still runs off and releases the Serpentine, which kinda sucks.
