I will honestly say that I did not expect Rock Bison to be on here, mostly because he hasn’t really done anything to deserve hate.Warning, there maybe a few spoilers ahead.
Useless superpower: How so? It wasn’t clear on what exactly his powers were, but from what is shown, it appears to be something of the lines of super durability, which isn’t a useless power. The guy can get hit by a full-speed car and shrug it off like it is nothing. Hell, the guy could probably shrug off bullets. Just because a power is not offensive doesn’t make it useless.
not good looking - This is purely subjective. Some people like the pretty boy types like Ivan and Barnaby. Some people like gruff, rugged and/or muscular men like Rock Bison. There is nothing wrong with finding certain type of people more attractive over other types. However, disliking a character based on aesthetics is NOT a reason to hate them.
is a joke character - again, how so? Because he has a few funny scenes? So do all the other characters! God knows how many times I laughed at Fire Emblem being overly flamboyant or Sky High being overly dramatic and I’m pretty sure there are plenty of people who found them funny too. Does that make them joke characters? Certainly not! Besides, he has his serious moments too. Look at the last few episodes, where he fought along with the other heroes to stop Maverick.
doesn’t really matter as Kotetsu’s “best friend” and gets replaced - Kotetsu doesn’t “replace” him. People have have more than one best friend. Kotetsu and Antonio have been friends since High school, they have history. The only reason why Barnaby gets more screen time is because Barnaby is one of the main characters. Antonio is a side character, along with Sky High and Dragon Kid and all the other guys. Also, if Antonio didn’t really matter, why did Kotetsu tell him to take care of his daughter if something were to happen to him in the last few episodes? Obviously he still thinks as Antonio as his friend.