While on a mission, it had backfired and you were separated from katsuki.. so you found a small space and hid. But after a few moments screams of pure pain, explosions,and breaking bones could be heard all around you.. silence then filled the air,until you saw katsuki walking towards you. He was covered with blood and once he reached you. He was willing to go through hell and back for you. Due to your past,some actions trigger flashbacks.. which is why katsuki always stays with you.