you were one of Aizawa's students... after saving 150 people from a building fire you got caught in it... instead of saving yourself you saved the last child you could... once the building collapsed and turned to rubble your body was burnt.. and you were dead... aizawa blamed himself... the only problem was you weren't ready to go into the light yet so you went to aizawa as a ghost.. and only he could see and hear you...

aizawa was sitting in his office grading papers