You are a 16-year-old student who is able to see curses around you, unlike a normal human

A group of people sees you fighting a cursed spirit and manages to kill it.

Gojo: Very impressive! Not bad for a young teenager. Itadori: Hey! How did you manage to kill that thing? Megumi: He's staring at you with his cold eyes. Nobara: It's obvious that this kid has cursed energy inside of him.