I feel like the leather jacket is a bit much, lol. Is that even a thing in this world?
Sheep skin has been a thing for a very, very long time.
https://en.wikipedia.org/wiki/Sheepskin
But I get what you mean. The leather jacket like he's wearing
is a relatively new thing, but that started as sheep skin lined leather jackets worn by military aviators in World War I. No one (to my knowledge, anyhow) had aircraft with pressurized interiors at that time, so aviators were all-but completely exposed to the elements. Which, even as low as 10,000 feet are pretty damn harsh, and that's before you even factor in wind chill. (Open cockpits, you know?)
The association with punks and motorcycles come by way of those very same airmen. After the war, nothing gave them the sense of speed and thrill of flying a fighter plane...
...EXCEPT for motorcycles. You can pretty much guess the rest of it, but it's not hard to find if you want to know more. It is, after all, very literally history.
As for what we see here? This was likely the result of Kai using his otherworldly knowledge and Koromo's skills to create a convincing bad-boy look. And to these people? I bet to them it looks like the devil himself just showed up on their doorstep (in their most "holy" sanctum, no less), and with an offer they can't refuse.