Joke’s on everyone because he actually starts responding to it at some point, much like the way he started responding to Denki calling him Kacchan. He didn’t want to, but it ended up ingraining itself into his subconscious anyway.

If they say it while he’s completely conscious, like “morning, kitty! Did ya sleep well?”, he’ll of course flip them off and go “eat shit and die”, but if he’s working on homework per se, completely focused, and they go “hey, kitty, can ya be a bro and help me with this question?”, he’ll 100% sigh, eyebrows still knitted together in confusion on the one he’s working on before looking up and going “sure, whatever. Which one you on?” “...”

It takes a lot of laughing and Izuku snorting for him to even take notice because no one tells him that he’s responding to the nickname so casually. It’s just too entertaining.

Not until Hitoshi and Shouto.

Katsuki was on his phone when they both saw a cat on school grounds and went “here, kitty, kitty, kitty” to it. Instantly, he complied and started walking over. He was confused when he put his phone away and found them both looking at him as if they were about to laugh.

They finally told him what happened, and he still hasn’t recovered from the embarrassment. Denki jokes about it to this day, making references like going “pspspspspspps” in Katsuki’s ear, and if he weren’t so goddamn cute, he would’ve probably killed him by now. But alas, Katsuki is one weak bastard when it comes to his friends.