I made this page 'cause a lot of people asked me "What happened to Kai when he was young?", "Why is Kai that cold?".Every time I answered not by the very same way...So, here I'll say everything I know about Kai's past.
 
 
 
 

When Kai was a little kid, maybe even just a baby, his parents sent him to his grandfather, the evil Voltaire. Then he placed Kai in his own Abbey (See picture above) where he was trained to be the best beyblader ever.That's what Voltaire hoped Kai would be. And he was right. His trainer was no one else than Boris himself. Kai had been trained all his childhood with no pity, in the dark. Even if Kai had suffered, Boris and the others wouldn't have mind. They would have force him to continu untill all the exercices are done.

In this Abbey, if any kid would show a sign of tiredness or ill, the staff would make sure he would not be tired or sick anymore. The little kid wouldn't come to the Abbey or even, he wouldn't live anymore...

Kai have been raise as a laboratory rat. He served for many experiences. But now, see the results! Kai is brawny, very well trained and well foster! But this also had some bad effects...Kai is solitary, cold-hearted...Well, Kai is Kai!!

Returning to Kai's past, one day, still in the Abbey, the scientists invented and created a powerful beyblade. That is supposed to be the best of all. Its name is Black Dranzer. All black, with a devastating strenght, it dearted on every blades on its way, while the first experience. Kai was one of the numerous kids allowed to look at the power of Black Dranzer. But he seemed to be the most interested by this strenght. He wanted to test it by himself. He couldn't sleep without thiking about it. So, the night after the strike of Black Dranzer, Kai succed in passing trough all the security and went to the beystadium where he saw the blade. He unhooked it from the automatic launcher. Kai had his own laucher with him. So, he puts Black Dranzer in it and launch...
Kai had all the skills to own it. But he didn't know how to use it.
So, when he launched it, he destroyed the half of the Abbey. Several kids were hurting and some died in the accident. But Kai wasn't injured, even though he was right in the middle of it. But his memory was tromatised by this. The scientists tried to make him remember, but they didn't succed. So they banned him from coming back to the Abbey. When Voltaire learned this news, he was a little bit frustrated to see his only chance of taking over the world, half trained. So he made him beyblade in the normal way, trough the tournaments and everything. He sent Kai at Japan, where he found some good bladers. They weren't the best, but with a little help with Kai's training worked fine.

During the series, at the very first episodes, Kai didn't want to talk to anybody and he wanted to battle for family honour.

Later, in the episodes between 41 and 51, Voltaire tried to lure Kai in his team, the Demolition Boys, with the powerful Black Dranzer. Kai was so tented of having the best beyblade just for himself, so he accepted and betrayed the BladeBreakers. They didn't really accept that Kai was gone for good, so they tried to change his mind and to re-enter him in the team.
Do you think Voltaire and Boris were happy about this? No, they weren't.When Kai lost his first match against Spencer in the Russian Tournament, Voltaire offered Black Dranzer to Kai once again. But this time, Kai refused. He knew that if he used it he would just serve the bad...And he didn't want, after all he had in his past.
 

RUMORS. Kai had his blue traingles 'cause of Voltaire, as a punishment. He forced to be tattooed(sp?) to show him that he was part of the Hiwatari family and that nothing could change this. So, this could mean that Kai didn't accept to be in the Hiwatari family...

But they seems more like paint 'cause once, in Beyblade 2002, we can see him without his blue triangles, at school. The students are not allowed to have marks like those on them.
 
 
 

That's all I know about Kai's past. I hope you had answers to your several questions.