Kai is one of the most popular and also among the strongest characters in Beyblade. He can be described as a character who is bold, strong and prefers to stay alone. Kai is a very capable blader and has a great mindset. He is able to accomplish feats that are extremely difficult for any normal person to accomplish.
In the beginning of the anime, he can be seen as an arrogant person who believes in destroying his opponent. This is shown through his very first battles when he destroys blades. He does form bonds with his teammates (Tyson, Max, Ray and Kenny) but prefers to stay alone most of the time. His cold and aloof nature is actually formed from his circumstances as a small kid.
His grandfather Voltaire Hiwatari owned Hiwatari Enterprises and his son Susumu Hiwatari(Kai’s father) left the enterprise to pursue beyblading. For this particular reason, his grandfather disowns Kai’s father which impacts Kai’s nature to a huge extent. Kai develops a hatred for beyblading as he feels it was the reason for his father to leave him.
Kai is also forced to train in an abbey in Russia by his grandfather and Boris. Boris and Voltaire run the BIOVOLT corporation where they train bladers to become cold and ruthless. The bladers are trained under very harsh conditions and are punished heavily for their losses. Boris probably believes that this is a way of producing great bladers in the sport which will prove to be a huge asset during tournaments. The Demolition boys are a product of hard training in the abbey. In the 3rd season, the Demolition Boys form the team called ‘Blitzkreig Boys’ to compete in the tournament and Kai joins their team since he has a bond with them.
Kai is very small when he trains in the abbey and he is attracted to Black Dranzer in the abbey which is supposed to be a really strong beyblade. In order to control the Black Dranzer, the blader had to be really strong in terms of mindset. Kai as a kid was not fit to be in control of Black Dranzer and he still had to pass a number of tests. For some reason, Kai leaves the abbey at a very small age and moves from Russia to Japan.
Kai has almost forgotten about his time in Russia, but the effect of his training Russia leave a huge impact on him as a person. Subconsciously, he wants to be the best at the sport and was willing to do anything in order to be so. When he goes back to Russia for the world championship, he is reminded of his time in the abbey through a lot of flashbacks that he gets while trying to recall his past. He creeps into the abbey out of curiosity to learn more about it and meets Boris who seems to be expecting him.
He accepts Black Dranzer as a bit beast and becomes ruthless in battle capturing bit beasts from other beybladers. Eventually, he is defeated by the bladebreakers who team up against him. The blade breakers also save him from drowning in the lake. So, that is the time when he finally undergoes a massive change in nature and starts following the right direction.
In the 3rd season, he joins BEGA led by Boris again but his reason to join them is only to match up against Tyson because he feels that is the only way he will get another proper shot against him. Eventually, he joins the blade breakers again and beats Brooklyn after loosing the first fight to him.
So, this is the story of Kai who in my opinion is the most interesting characters in the series.