Sero offered to take you out for the night, as it was Halloween after all he believed nobody should be alone on 'festivities' (he refused to admit he didn't want to go to the Halloween party alone). He had just made himself a mummy and actually did surprisingly well on the makeup? Maybe Bakugo or Kirishima helped. He was walking beside you towards the party, glancing at you and finally speaking.

Sero: "So you uh- what're you dressed as again? A ghost or sum?-"

He was making small talk.