Kai's character design is just a true flex for Dorohedoro. It's got pretty indulgent character designs overall - badass looks are not reserved even for foreground characters. It's a universe where fashion deviance is so normal it becomes noise. Similarly, spectacular gore and violence are used liberally, in both major plot beats and one-off jokes. On paper, this makes a character like Kai impossible to distinguish from the rest of the cast. It matches the makeup of a Hazbin Hotel type situation, in which every character is befanged, red-eyed, and maximally accessorized such that none can actually Be Sinister in a way that stands out.
Dorohedoro dodges this trap through the raw power of good writing and panel to panel composition. Kai is a plain character. He does not have a "look" that isn't already established (minor characters have cross eyes, many wear dark clothes). He's Big and he Kills People, on paper not much different than Shin and Noi, or even Ton. But Kai is written (and lit, and paced) to have gravitas. The paneling pauses on him, bends around him, to make sure that we understand the difference.
When Kai kills people it is more awful, more violating. Gore around him is never comedic. Much like the classic slasher, even just standing and walking looks evil on him. A character like Shin is similar in profile, but a known quantity in intentions. Shin will kill people for his boss, and for Noi, and maybe for fun. This de-mystifies him - Shin can kill a character you'd prefer he didn't, but after his introduction he is no longer really "scary". Kai is a black box, who kills or spares or stands or watches for reasons we cannot know. Chidaruma lolling his tongue out is ridiculous, meant to demean his own character, but with Kai it is meant to unnerve. And it does.
In a world so uniformly full of slasher aesthetics - the clothes, the weapons, the kills - Kai is the only Real Slasher.
