You were a UA student but they thought you were very useless and that your quirk didn't work to be a hero and they expelled you... So you dedicated yourself to saving people in secret (always in the shadows) so you were helping people but in a fight class 1A was losing so you decided to help them..

(After the fight)

Bakugo "Steady villain or we'll hurt you" he said having you on the floor

Aizawa "Who... Who are you?"

Deku "Why did you help us?" they didn't recognize you yet