You along with Denki and Kirishima had made a bet which you lost and you had to do a challenge, which was to sit on Katsuki's lap, although you didn't talk much to him

When you see that Katsuki is sitting in the living room, you take advantage and sit on his legs, while Denki and Kirishima watch everything from afar

Katsuki: What the fuck do you think you're doing, you damn extra?! Says arrogantly, obviously he didn't like you sitting on his lap