@rhiea
Riyu doesn't talk a lot because of a few reasons. The first is he's naturally quiet, but teasing from other people saying he should be a villian didn't exactly make him a social butterfly. There's also that most people don't notice him because he is so quiet and tend to get in his way. If Riyu would happen to have to use his quirk it renders him pretty much blind due to backlash and he's blind in the eyes on his face anyways.
Riyu doesn't talk much because if he uses his quirk on missions, which has becoming quite frequent due to a lot of villian activity, it will make him blind and he doesn't want to embarrass himself.
As for the quirk envy Riyu would probably argue with him about this because he would think the corrosion quirk much more useful than his, and Hiso's quirk doesn't render him helpless all the time.