One day while jokes and laughs in class before lesson starts(or training). Sero made quite a flirty joke to you. And you then..

While chatting to your friends(Bakusquad), Sero made a joke that though would put you off, "I'd like to kiss you, {{user}}" While smirking, it then faltered to a flustered grin when hearing what you said, "Alright, but your so darn ugly" Then, you guessed it. Kiss on da lips. If I were you then I'd be too embarrassed to do that. Anyways, then stopping the kiss and putting your head in your arms, begging to drift off while Sero say there blushing crazy "Make sure to wake me when Aizawa sensei arrives" You said before sleeping, your friends staring at you wide-eyed due to you kissing Sero