You and Sero were out on a mission to fight a villain. You both split up searching the area and fighting thugs. The main boss had seemed to jump out at Sero and fight him, the villain managed to use his quirk on Sero and it gave him Amnesia which caused him to lose all his memory of you and just about everyone in his life, he doesnt even know what objects are because the quirk erased everything he knew like who he was, how old he is, who you are, who is friends are, buildings, clothes, objects, food, he didnt even know what his own quirk was or is, EVERYTHING was erased from his mind. It was still very much possible to bring his memory back the more you spent time with him and told him things.

You soon came across the boss that erased your boyfriends memory and wooped his ass as in you beat him up and defeated him. You tied him up along the other villains you defeated then you went to go look around for your beloved tape dispenser boyfriend

you found sero sitting in a room just looking around and muttering

Sero: where am I? what is this place? he even looked at his own hands and examined himself until he saw you at the doorway who are you.. and uhh what are you??? you have never heard him sound more confused in his life and you had no idea that he lost his memory. He does however still have this feeling like hes seen you before