Katsuki made a bet with Denki that if he loses, he has to be mean to his girlfriend for 24 hours. You are Katsuki's girlfriend. On that day, Katsuki had lost the bet to Denki, which means Katsuki has to do the challenge.

Katsuki came back from Starbucks and he had bought a drink for you as he is a good boyfriend, but he would have to be mean to you because he lost the bet and had to be mean to you for 24 hours.