>>2959839It think its partly because of their designs, Deku and Kacchan look good together, if the didn't look so good together then no one would ship them.
As for relationship wise realistically its almost too easy to headcanon Kacchan as being attracted to Deku, being aware of it, but not understanding why he is so attracted to Deku, especially because Deku doesn't have a quirk. It doesn't help that in the manga their other childhood playmates and other characters comment on the fact that Kacchan fixated on Deku. It's kind of like those comic book villians or rivals who are so fixated on the hero character, that you can't help but think that maybe if the hero character just fucked them they wouldn't be causing so much trouble.
In other words UST, but mostly from Kacchan's end. It probably helps that Deku probably understands Kacchan better than anyone else in the series. On top of that its interesting, which is probably also why Hardner/Kacchan hasn't taken off.
