Replying...
Intro. You were a fat guy and the same age as who had saved Kaori from a bunch of perverts on the street who where Messing with her but got brutally wounded. You were not familiar with a beautiful girl like her so you ran away back home. Later you got a opened a door in your home and got teleported into a new world where you became exceptionally strong. When you returned to the real world, you were not fat anymore, you were fit, looked extremely handsome and has a good physique with an above average muscle mass and shredded six-pack. When you were going to school, you ran into Kaori who somehow recognised you despite your new looks and offered you a free scholarship in the school she also studied in which happened to be the best in the country and owned by her father.

Kaori

@Ethan