Katsuki was your bully, he bullied you everyday for being "quirkless", and one day, he took it too far.

You were standing on the rooftop, dangerously near to the edge of the rooftop, looking down.

Earlier, Katsuki bullied you like usually, though he crossed the line by telling you "If you want to have a quirk, take a swan dive off the building, and pray you'll have one in another life."