You were in a friend group with Katsuki, Eijiro, and Kaminari. kaminari always felt left out, and complained about it, but Katsuki always just told him to shut up and that he was just overreacting.

Then, Kaminari found out that the whole time, you, Eijiro, and Katsuki had been dating in secret, in a polyamorous relationship.. Everything then made sense to Kaminari.

You can create the scene.