Kai Chisaki is a Pureblood wizard that was born on the 20th of March 1957 and started attending Hogwarts on 1st of September 1968, being sorted into Slytherin House.
He has a Yew wand with a Dragon Heartstring core.
His Patronus was non-corporeal.
His favorite subject was Charms and his least favorite subjects was Herbology.
Kai is a mysophobic, antisocial sociopath obsessed with reshaping the world to one without magic. Due to a combination of an old theory claiming magic derived from rats and his mysophobia, Kai believes that magic is a plague on humanity, having infected people with "syndromes" of light and dark. He looks down upon those that use their magic for either purpose, referring to acts of heroic sacrifice or petty crime as "sicknesses". Kai is highly determined to his ambition of eradicating magic, not only due to his phobias but also because such a thing would allow a Mafia Boss to reclaim the power they once held over society. Kai has no moral compass and an enormous ego, viewing himself as the only person in the world who's enlightened to the "true nature" of magic and deserving of the spot as ruler of the underworld after All For One's downfall. Despite this, Overhaul's values are noted to be paradoxical, particularly his over reliance on his own magic, which he happened to name himself after.
As the leader of the Shie Hassaikai, Kai is usually levelheaded, thinks strategically, and, according to a conversation with Twice, seems nice at first. Whether he is acting or behaving genuinely, Kai possesses polite mannerisms and can come off as very calm and classy even to people he considers potentially troublesome, evident during his first meeting with Mirio Togata and Izuku Midoriya. If aggravated, however, he may display a powerful and visible killing intent, more fitting of his true character.
Kai is very germaphobic and hates anything he perceives as unsanitary, frequently remarking about the unclean appearances of certain people and locations. He refuses to be touched by others or even breathe in the same air as theirs, which is one of the reasons he wears an air-filtering plague doctor mask and forces his subordinates to do the same while around him. If blood, dirt, or some other unclean substance comes in contact with Kai's person, he will start developing hives and lose his usual composure, becoming increasingly unhinged as a result. In drastic circumstances, though, Kai will let go of his aversions to crush an enemy standing in his way, either due to pragmatism or out of sheer hatred.
Kai doesn't value humans, viewing people as pawns for their utilitarian value and being willing to experiment on them to accomplish his objectives. He carries this sentiment even towards the other members of the organization to which he has dedicated himself since childhood, treating them as expendable sacrifices for his well-being and having no qualms about killing them off for the most minor mistakes. The masks worn by Kai's closest subordinates also serve as a reminder of this mindset; he doesn't view them as worthy of sharing the same air as him.
Even Eri, his boss' granddaughter and the centerpiece of his operations, is not spared from this brutal way of thinking as Kai frequently shames and abuses her through threats and guilt trips, seemingly to force subservience on her mind. He ruthlessly performed experiments on Eri, which involved painfully disassembling and reassembling her body with healing charms, and had no problem with killing people in front of her, including her minders when they allowed her to escape. Kai was possessive over Eri due to her being the key component in his plans, flying into a massive rage when she was taken from him and going to extreme measures to get her back. Despite being assigned as her foster father, Kai had no intention of parenting her and later confessed to Mirio that he never saw her as his child. Another hypocritical facet of Kai's plans was his obsession with Eri's powerful magic, the power that enabled him to fulfill his plan of destroying other people's magic.
In public, Kai acts as a strict yet regular father, becoming irritated when bystanders pry. He casually made excuses for Eri's bandages and fearful behavior, saying that she was simply a clumsy and badly behaved child who was afraid of being told off.
The only person Kai has ever shown genuine care and concern for is the former Shie Hassaikai Boss, who rescued him from the streets at a very young age and raised him as a criminal. Kai, to this day, still feels deeply indebted to his boss for taking him in, being adamant in returning said debt by returning the organisation to their glory days somehow. Ironically, despite his sociopathic mindset and inhumane treatment of others, this makes his ideals about family much more personal than any other members of the organization. Aside from this, he has also shown to value Hari Kurono, his childhood friend and right-hand man, having healed his injuries and being the only one of his subordinates he does not treat as disposable.
As a young wizard, Kai was extraordinarily ruthless and would kill anyone who didn't show the Shie Hassaikai the proper respect, constantly getting into fights with rival gangs. Kai's boss, who was growing aware of the violent, immoral path his underling was following to uphold the name of the Shie Hassaikai, attempted to sway him towards a more honorable course, but to no avail; Kai became more and more convinced that illegal, questionable dealings were the only way the mafia could ever restore their former honor. This immorality would lead the misanthropic Kai to develop an interest in the boss's granddaughter because he considered her rewinding abilities a golden opportunity for the Shie Hassaikai, believing that he could produce a Magic-Destroying Drug out of her cells that would give the mafia enough power to take control of the black market.
Despite his loyalty and indebtedness to his superior, Kai could not accept his boss's refusal over the idea, attacking and leaving him comatose to work on his plan unimpeded. Kai took over the Shie Hassaikai under the twisted belief that everything he was doing was for the sake of his boss and the organization, intending to bring his boss out of his coma after he completed his plan.
Ultimately, Kai's failure to recognize others and his obstinate nature resulted in his downfall. As noted by Tomura, even though Kai's goal was to erase magic and restore the mafia to power, he hypocritically used magic to achieve that plan. Despite his strong-willed attitude, he is capable of expressing genuine fear and shock, especially post-defeat, after the League of Dark Wizards took away his product and ability to use his magic, screaming as the reality of being unable to bring the boss out of his coma hit him.
As seen by the time of Azkaban's mass riot and breakout, Kai's imprisonment and the comatose status of his boss have left him as a shell of his former self, being rendered to a near-catatonic state and being unresponsive to everyone, anyone, and everything around him. This refractory state is seen when he had little to no reaction to the mass riot during the Azkaban prison break and later showed no response to the appearance of Tomura Shigaraki and All For One, only being able to mumble the words "Boss", in reference to his father figure. During his talk with Lady Nagant, Kai laments over having lost everything, not even wanting to exact any revenge and only cares about reuniting with his boss, mumbling about how he wants to see him again, even when close to danger.
Kai is even shown crying out of regret while begging to apologize to the boss. While he still hates Izuku (despite the latter having saved his life) and holds no remorse for his treatment of Eri, he does agree to make amends with Eri, but only so he could save his boss. This would ultimately not happen however, in order to spare Eri from reliving her past trauma.
Following the Final War, Kai finally reunites with his boss who had been awakened through advanced support items. In the face of his former boss, who admonishes him for his crimes and for causing Eri so much pain, all Kai can do is cry, snivel, and apologize, with his inhumane actions having doomed both himself and the organization that he swore to revive.