Kai is a massive and imposing man with tan skin and short, spiky black hair. Visually, he looks identical to Aikawa but with the addition of dark cross-shaped marks over his eyes which are noted to be almost like birthmarks.
His typical outfit consists of a black hoodie with a rib-like adornment around his waist. He also carries a large holster with his trademark knives.
In the early days of his existence, Kai appeared to be a dour and extremely serious man of few words. He was overall extremely apathetic and rarely smiled. Despite this, he was also intelligent enough to found a gang and teach each member how to fight back against Sorcerers with mere knives, and blossom it into a blooming black powder distribution organization. Kai was also a ruthless gang leader, murdering and taking the heads of Sorcerers without any second thought.
Despite his laconic attitude, he seemed to care to some degree, shown by the way he took in the original Cross-Eyes founding members as young kids. Furthermore, despite his large appetite, he still shared some of his food with Ton simply because his stomach growled (though he did just throw it at his head wordlessly).
Later when he makes his return, he is notably more unhinged, hardly ever speaking and with a near-consistent mad grin on his face. The influence of the dark "entity" within him appears to be much stronger, rendering him something closer to a mindless monster acting on the murderous whims of what created him, to begin with. Unlike before with the original Cross-Eyes, he no longer shows any semblance of kinship nor cares for those around him, willing to murder anyone for his own benefit.
Kai was initially created when Ai Coleman dove into the Lake of Refuse to retrieve the body of a Magic User whom the militia had purged. Unbeknownst to near-all, the Lake of Refuse was the culmination of millennia of built-up resentment of humans who suffered or died at the hands of Sorcerers, simply known as Hole. As Ai's body was eaten away by the toxic waste of the lake, the resentment was absorbed into him. After Ai survived due to Dr. Kasukabe's surgery, the presence remained and festered within him.
After Ai was buried after his murder at the hands of Sorcerers, the presence of Hole revived his body, and combined with part of Ai's original personality, became "Kai". The newly resurrected Kai wandered for a while until he came across a magic door opened from the Sorcerer's World, with some Magic Users dumping five kids to die in Hole. He ruthlessly kills the abusers with a kitchen knife, and with the kids -- Dokuga, Tetsujo, Saji, Ushishimada, and Ton -- in tow, entered the Sorcerer's World. There, they formed the basis of what would become the Cross-Eyes Gang.
During his time as the leader of the Cross-Eyes, Kai used the knowledge attained by Ai's personal research and apprenticeship under Dr. Kasukabe to great effect, first perfecting a technique that used simple knives to completely disable Sorcerers by targeting their smoke organs. Furthermore, he, and subsequently the Cross-Eyes became well-known for their M.O. of removing their victims' heads, which Kai then dissected to take out their devil tumors. With it, he created Black Powder, a steroid-like substance that amplified the magic of weak Sorcerers.
Many weak Sorcerers who were bullied by stronger and more powerful Sorcerers came to revere Kai and the Cross-Eyes, and they gradually expanded into an organization. Their defining trait being the cross-shaped tattoos they had placed over their eyes to mimic their boss. Magic Users began to fear the Cross-Eyes for their growing influence and ability to seamlessly slaughter them.
In reality, Kai was driven by Hole's agenda, which desired to kill all Sorcerers, and/or steal their magic for himself in the process. Though he mostly came off as a serious and dour boss. His sinister side occasionally showed, such as when Saji walked in on him obsessively counting how many devils he had ripped out of the heads of Magic Users.
Eventually, he targeted the early En's Family members, killing them one by one for two years until En himself took care of the matter. He decided to send Shin and Noi to Kai's hideout in Mastema where he had already killed Tenjin, one of the founders of the En's Family, by chopping his head off and using it as a decoy to lure the cleaners to a corner. Kai almost killed both of them until En showed up to fight him personally. Kai's Hole powers emulating the rain effects of the human world made En scared and weak, and only by transforming absolutely everything in the city of Mastema into mushrooms he and the cleaner could get away alive.
Kai escaped the showdown by switching, making his head, thereby inexplicably making him immune to En's magic. The sight of the beheaded body of Kai walking through the door traumatized En for years, prompting his desire to find a sorcerer able to use time-magic, to go back in time and see if the Cross-Eyes Boss survived. Kai kept a low profile, continuing his research and leaving the organization management to the Top officers, with Dokuga as his sole link.
Two years before the start of the series, Risu, a newly enrolled Cross-Eye, discovered the reason behind his lack of magic smoke using a device designed for defining the kind of magic one has. He quickly told his friend Aikawa (the personality with which Kai shared the body) about his Curse-type of magic, an extremely rare and powerful one.
Kai used his other personality's memories to track him and set a trap in Salmanazar Field, where he killed Risu to ensure his power for himself. This unleashed Curse, Risu's magic avatar that started to destroy everything near it to find and kill Kai. The Boss avoided Curse for a while, but he knew the monster could sense where he was all the time, following him to every place he went and killing anybody who dared to attack it. With the help of Dokuga, Kai took measures to escape from Curse until he managed to come up with an idea to counter it. He entrusted Risu's chopped head (Kai was going to transplant the deceased Cross-Eye's Devil Tumor to his own head to ensure his power) to him and opened a Magic Door to Hole, but at the last moment, Curse went through the Door's lock and pursued Kai mercilessly until it caught him.
Kai's head was torn apart by Curse. The creature eats it before entering The Boss's body through his neck to consume him. But, before that could happen, Kai crushed a bottle with Ebisu's smoke, one of the many bottles of smoke Dokuga gave him as a help to escape to the human realm. Her magic, Curse's magic, and the Hole influence inside his body canceled each other out, rendering his body invulnerable to magic. As a result, he imprisoned Curse in his mouth, making him amnesic in the process and morphing a new head with reptilian features. Thus, during that series of events, Kaiman was born.
Once the spell on Kaiman's head was broken, with Curse freed from his body and becoming Aikawa once again, Kai reappeared, taking possession of the body to kill En in his Mansion and take his devil tumor to be able to use his magic. After a self-performed surgery in his old apartment, Kai was lying in a bed covered in blood. But he was healed by Natsuki who used heal-type smoke. The Cross-Eyes top officers and she brought once they heard the news about his return and the death of En. This was just in time to save the top members of En's Family, the cleaners Shin and Noi, expelling a large amount of transforming-type mushroom smoke (more powerful than En's magic used to be). They beat them almost to death before escaping thanks to the intervention of Shou, an old assassin of the En Family.
He and the rest of the top members took over the Mansion to use it as their new headquarters, exterminating almost all the rest of the En Family members left in the Mansion. And with that, the Cross-Eyes became once again a powerful group, with tons of new recruits and more resources than ever before. But something changed in Kai, now more violent, mindless, and inhuman in behavior. Showing more of Hole's possession over his body, he now had blank eyes and a constant evil grin on his face (in contrast to his old, perpetual serious face). Scaring everyone near him and concerning Dokuga, who knew he was Risu's killer. Having shown no remorse for killing his own comrades before, he slaughtered Natsuki after Aikawa (once again in control of their body), witnessed her type of Magic, to Dokuga's horror.
Using the time he had possessing Aikawa's body, Kai started his research again. But now with more resources than ever before, making black powder and launching a full-scale Magic-User genocide. He went to one of their raids to kill sorcerers, and the massacre went on for days. Even his closest friends started to think this was madness, slaughtering hundreds, even thousands of Magic Users, and filling the streets with blood and beheaded corpses. Though attacking and murdering every single Magic User they found, powerful or not, made even Dokuga sick. (He puked in a corner after seeing so much death, guts, and blood spilled all over the place.) The Top Officers followed Kai without hesitation, believing there was a reason behind this.
They attacked Tanba's restaurant. Still, thanks to the combined efforts of Fukuyama and Kirion, the former drove away from the Cross-Eyes while the latter chopped Kai's head off with a butcher knife, killing him and triggering his resurrection once again. The body is now being controlled by Aikawa once again. He shouted at Fukuyama to turn him into a pie to stop Kai for good. The sorcerer understood and transformed Aikawa into a giant meat bag, and the Top officers took the body (and a partially transformed Saji) back to the En's Mansion.
Now revealed that the purpose of Kai's plan had been to collect enough Devil Tumors to create a bodysuit made of Magic User flesh. His supposed goal was to become a Devil by artificial means to be able to reach Hole and give his body as a "sacrifice" to the Entity inside him.
During the events inside the Central Department Store, Kai ascended to devil hood and used his cast of artificial skin to turn all the humans in the store into zombies. He became as powerful, strong, and invulnerable as a true Devil, and used a possessed Shin as a pawn to get rid of any other person remaining in the store who would pose a threat to him. He unsuccessfully tried to kill Nikaido using the Store's Kitchen Knife she dropped thanks to Kai's power to emulate Hole's rain. At that moment, he was killed By Ai Coleman, who (using the second Kaiman body) chopped his head and arms off but was unable to permanently kill him (because he still had two heads left). Now headless, he took Nikaido and her Magic Time box and headed to the chamber where all the remaining Magic Users were. Kai battled En's mushroom clones in the room of Ai's mind and was killed once again by an overdose of mushrooms that had grown inside him until all his bodily functions stopped.
Once they arrived at the room, Ai used the Store's Kitchen Knife to cut the morbid deformed head of his evil persona, triggering the final resurrection. With it, the last remaining head. At that moment, Ai knew that with no other heads to regrow except his own, he abandoned Kaiman's body, and his old, rotten, and disfigured head appeared from the neck of Kai's corpse. With his final resolution, believing this would be the very end of all, he asked Risu to kill him using the Knife so Hole would no longer exist. Hesitant about the idea of killing his only friend, Ai/Aikawa told him that if he has to die, he would prefer to die at his hands, after a sad goodbye between them, Risu sliced Ai's head from the neck of Devil Kai.
Risu talked with the still alive chopped rotten head of his friend, Ai lamented to be nothing more than a tool for Hole, and his last actions gathered the most powerful remaining sorcerers in the chamber. Kai resurfaces once again for the last laugh, telling Risu that they are all doomed. Now that everything is settled, the vessel that was Ai Coleman, and his heads are no longer needed. Infuriated by the idea of killing his best friend didn't put an end to that nightmare, Risu throws Kai's head to the ground, stomping over it with his foot, killing him for good.
- The kanji for Kai's name, 壊, means 'destruction'.