I really don't have any evidence to support this headcanon, but I like to believe that Kuya is blind in the eye that has the gem. I'm not to what extent he'd be blind, whether his vision would be extremely blurry, completely nonexistent, or something else, but he's blind in that eye

Luckily, he's had plenty of years to learn how to get through life with only one seeing eye and now he sometimes forgets that he's partially blind. On top of all that, foxes have an excellent sense of hearing. So whatever his vision doesn't get, his hearing does