This post explains my own headcanon of what Kai went through as a child to shape him into the teenager he is today. If there's anything that seems alarming to you, or unjustifiable, please feel free to contact me on the information listed in the profile!
Anyway, let's get this show started.
All right, well, Kai has not had a good childhood. This is pretty much proven in the beginning of the first season and somewhat in 2002/V-Force. At any rate, Kai's mother died when he was very young, and his father went away, so that left him in the hands of his grandfather, Hiwatari Soichirou (Voltaire), who was not the guardian he should have been to Kai. Rather, he told him that he had to do nothing but win, win, win at Beyblading, and as such, Kai went with his wishes and started to learn about Beyblading to be the absolute best the world ever came to know.
Yet, like everyone, Kai made mistakes and his grandfather made sure to admonish him for such by striking him as well as mentally abuse him. When Kai's grandfather saw that putting him under his wing wasn't enough, he sent him off to Volkov Abbey, a monastery or a church in simpler terms, where it was there he was trained and turned into one of Volkov's "perfect soldiers" to dominate the world with Beyblading. It was here the Abbey took Kai's passion for beyblading and warped it into making him a Beyblading machine.
In the Abbey itself, Kai was put under brutal conditions. They either slept on cold floors, or they had a mattress underneath them, but that was it. They had food, but it wasn't the greatest food known to man -- it was cold soup and bread, enough to fill their stomachs. Now, due to being part of the Hiwatari family, a lot was expected out of Kai and that was to continue to win, win, and win. So, whenever he made a mistake, Volkov, or one of the scientists hit him, punished him, yelled at him... which only made the darkness in his heart bigger and wider. Kai had no need for friends or acquaintances, since those people would just get in his way.
However, that all changed when he met up with one Yuriy Ivanov and Boris Kuznetsov in the Abbey. There was something that Kai saw in the boy that made him want to protect him, so he did whatever it took to make sure that Yuriy's and Boris' time in the Abbey wouldn't be as bad as his time was. This meant harsher punishments for Kai, but he wouldn't let any harm come to the two of them, and when it unfortunately did, Kai took it upon himself to treat Yuriy's and Boris' wounds. The three of them were close -- almost like brothers, who always had the other one's back.
Kai ended up leaving the Abbey after the explosion from Black Dranzer, which incidentally left him in shock and with temporary amnesia. He was sent back to Japan with his grandfather, who took advantage of the situation his grandson was in and filled his head with lies that him and Yuriy were great enemies, among other things that only skewed his perception of how he viewed the world. Kai took this time to form his own team -- the Shell Killers, who terrorized and stole the losers' beyblades. He hung out in warehouses and knew his way around alleyways. Now, this kind of gang lifestyle for Kai surely had its' drawbacks. He did end up getting hurt, and he did end up making some enemies along the way who wanted his head, but it was with this lifestyle that Kai knew how to survive on the streets and steal things to get by. Nothing really mattered to him but the sweet smell of victory. Throwing a few people under the bus? Sure, why not. Manipulating and talking shit about a handful of people? Fine with him. As long as he won in the end, he could take whatever people threw at him and still stand strong against them.
The memories returned when Kai went back to the Abbey, only to realize then that his home was the Abbey all along and that Yuriy was his best friend all over again. He only changed when he almost died from drowning at Lake Baikal due to the determination that Takao had to save his life and to realize his captain duties to Team BBA.
Now, Kai took some time off from Beyblading to attend a prep school, which constitutes as his
Ten Minute Retirement, since recovering from almost drowning took a lot out of him. His spirit for the sport was regained when he saved Yuuya from an attack, and therefore returned to his sport to kick ass and take names. When he was fighting Team Psykick with his own Dranzer, Kai had to tell himself plenty of times that it wasn't Yuuya he was fighting -- it was Goki. Now, if there's something that Kai cares about -- it's his beyblade and his holy beast, so when you take one of them away, he will get pissed and he will get even, since anyone that's known Kai can tell you to never get in his way because he will get pretty nasty pretty fast. However, he didn't get Dranzer or Suzaku back until Takao got everyone's holy beasts back to their respective owners.
To me, Kai in G-Revolution had the most development. Everything that he went through made him realize his true potential and also helped put a few past demons to rest. Yet, it was also the season where Kai knew he had to defeat Takao once and for all. So, this meant that he left Team BBA to rejoin his old team, Neoborg/Blitzkrieg, which was wrestled out of Volkov's control. He teamed up with his childhood friend Yuriy to reach the finals, where the two of them are nearly unstoppable in battle, due to the power they have. While Yuriy took care of Daichi, Kai faced off against Takao. The battle was so intense that the arena around the two of them started to crumble, meaning that at his highest power, Kai is a formidable force. However, Daitenji-kaichou told them to pretty much throw in the towel and call it a tie. Now, Kai has not, will not and won't ever give up, no matter the cost to himself to win and to conquer an obstacle in his way. So naturally, he cursed the old man out to let him and Takao finish their match, which did end up in a loss for Kai due to him collapsing after giving it his all.
Some time after this, we don't see much of Kai. I've thought about it, and Kai took that time to train and to get better so that he could eventually defeat Takao once and for all and reclaim his World Champion title from him. When Yuriy went up against Garland of Team BEGA, the way he saw it is that while he was searching for his strength, he couldn't go back to Team BBA just yet. Despite it all, Kai, while he was on BEGA's team, knew how to play his cards right while on this team, which was almost like a betrayal to those who were his fans. He ended up disappearing for a bit, however when we see him again, he goes up against Brooklyn, someone that reminded Kai of himself with how cocky and arrogant he was. Kai fought his hardest, but ended up getting his ass kicked by Brooklyn, which ended up in him becoming broken from this loss. He spent his time wandering through the alleyways, taking care of stray cats, and trying to not get too much attention to himself. Why? Kai couldn't bare to let anyone see him at his lowest due to him being too prideful about it. He took years to create the feared beyblade machine that he was, and to let anyone see him in this state was a huge embarrassment, so he isolated himself from anyone and everyone.
That was, until he went to visit Yuriy in the hospital, in which Yuriy was knocked into a coma after his battle with Garland. To him, Yuriy was the one person that understood where Kai was coming from. When he visited Yuriy, one of the nurses said that visiting hours were over. It was this that made Kai break his silence and say that he wasn't leaving. After watching over Yuriy, Kai noticed that Dranzer was left behind for him. Surprised, he grabbed his beyblade and left the hospital to hone in on his skills so that he could defeat Brooklyn and his own past demons once and for all. Of course, with Dranzer MS, his new beyblade took a bit of time to master the power it had, yet he was determined to win and to get this entire thing over with. With Romero's help of him realizing his true potential, Kai went to rejoin Takao's team after seeing them trained to the bone by Takao.
The next match that Kai had, he came back to the beystadium, still adorned with the cuts and scrapes he received from the last time he went up against Brooklyn. The entire crowd that gathered for this called Kai a traitor for walking away from Team BBA. However, Kai paid them no mind since he was set on a goal and had to see this through to the bitter end. This fight he had with Brooklyn was his most challenging and also painful battles because of the amount of raw power Brooklyn had. Yet, Kai did not give up at all, even as he took blow after blow, hit after hit, even though he was steadily getting worse as the battle went on. Even after passing out a few times, Kai refused to let himself give in this easily. He looked inside of himself to find the strength that he knew he had to feed that into Dranzer to take on Zeus, who wasn't doing well due to the amount of damage it took. As the battle went on and on, Kai saw the light at the end of the tunnel, took the opportunity by the reins and defeated Brooklyn all by himself, even as his teammates, sans Takao, wanted him to give up and interrupt his moment.
After this fight, Kai came out of the stadium, bruised, beaten, but still managed to keep himself standing. He collapsed into Takao's arms, only to force himself to stand up, wave his teammates off by saying that he was fine and that Takao owed him a rematch. When he was alone, Kai finally felt the toll the fight had on him, slipping to his knees as he let go of Dranzer MS, it completely shattering as he slipped into unconsciousness. It was after this that Yuriy, still in his coma, shed a tear for Kai. It's speculated that this either meant that Yuriy was proud of Kai, or that he died, or came close to death. One of the tournament officials found Kai in this state and rushed him to a hospital, where he had surgery done on his eye, as well as on his chest and was pretty much covered in bandages head to toe. As Suzaku returned to him, due to her being a
vermilion bird, to me, she brought Kai back so that he could lend his strength to aid Takao against Brooklyn.
As Kai watched over Takao battling his hardest to save Brooklyn from himself, he was proud of the other for not giving up in his goal. As Brooklyn was defeated and the spirit of beyblading returned to everyone, as well as Volkov's plans ruined once again, he was happy to see that old man persecuted for his crimes. So happy that Yuriy and he shared a smile together, and went on their way to make up for lost time. Some time after this, Rei, Max and Kyouju were training new beybladers the way that beyblading should be -- fun, challenging and entertaining, not the professional way that BEGA and Volkov had it be. Kai lounged with Takao on a grassy hill, near a playground, then the two had their rematch where the results of the battle are unknown. After this, Kai went on his way to lay low for a while and just enjoy life at its' fullest, keeping in contact with Takao, Rei, Max, Kyouju as well as Yuriy, Boris, Sergei and Ivan, even taking a few trips to Russia to see his old team again and to spend some time with them.
AND THEN HE WAS TAKEN TO DISCEDO. THE END.