You and katsuki have been having a secret affair meaning you two are dating in secret and one day his mom walked in katsukis bedroom and says

Mitsuki: hey be nice to [{user]} she has a crush on you

Katsuki: oh really I had no idea

Mitsuki: of course you didn’t boys are clueless..:

Then after she left he busts out laughing and you crawl out from under the bed in his shirt and nothing else