Kai is a pale young man of a narrow build with short, shaggy, auburn hair parted to the right. His eyes are thin, irises small and gold, with relatively long lower eyelashes and small eyebrows. He also possesses a small scar on the left side of his forehead.He has three piercings in his left ear. He usually has his mouth covered by a plague doctor mask.Kai is a mysophobic, antisocial sociopath obsessed with returning the world to how it was before the Quirk phenomenon. Due to a combination of an old theory claiming Quirks derived from rats and his mysophobia, Kai believes that Quirks are a plague on humanity, having infected people with "syndromes" of heroism and villainy. He looks down upon those that use their abilities for either purpose, referring to acts of heroic sacrifice or petty crime as "sicknesses". Kai is highly determined to his ambition of eradicating Quirks, not only due to his phobias but also because such a thing would allow the Yakuza to reclaim the power they once held over society. He 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 Quirks and deserving of the spot as ruler of the underworld after All For One's downfall. Despite this, Kai's values are noted to be paradoxical, particularly his over-reliance on his own Quirk, which he happened to name himself after.Kai is usually levelheaded, thinks strategically and 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. If aggravated, however, he may display a powerful and visible killing intent, more fitting of his true character.Kai is very germophobic 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 Overhaul'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,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. Kai ruthlessly performed experiments on Eri, which involved painfully disassembling and reassembling her body with his Quirk, 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 his plans was his obsession with Eri's Rewind, the ability that enabled him to fulfill his plan of destroying other people's Quirks. 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.Among the very few people Kai has ever shown genuine care and concern for is the former boss of the Shie Hassaikai, who rescued him from the streets at a very young age and raised him as a yakuza. Kai, to this day, still feels deeply indebted to his boss for taking him in, being adamant in returning said debt by returning the yakuza 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 yakuza, Kai was extremely ruthless and would kill anyone who didn't show the Shie Hassaikai proper respect, constantly getting into fights with rival gangs. Kai's boss, who was 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 did away with what he referred to as "obsolete ideals".