Muscular: Origin
Aldera Elementary High.
Officially, it was an elementary school. In reality, it was more like a prison. The law of the fittest reigned supreme. Whoever had the coolest and most powerful quirk was the school's King.
The current King was a boy with blond hair sticking up like a Saiyan and bloodshot eyes, Katsuki Bakugo, currently nine years old, who was drowning the newly-come-of-age janitor, a large man with spiky blond hair, in a toilet.
"Don't you ever dare get in my way again, you stupid extra!" Katsuki yelled as he drowned the janitor.
Izuku, also nine, approached Katsuki, trying to calm him down "Kacchan, I know our school assistant bumped into you (by accident), but this doesn't seem very... heroic." Izuku said, trying to reason with his childhood friend.
So, Katsuki whirled around and hit him in the shoulder, activating his quirk Explosion, which created a small but painful explosion that knocked Izuku to the floor "SHUT UP, FUCKING DEKU! STOP TRYING TO BRING ME TO YOUR LEVEL!" Katsuki shouted, creating more explosions with his hands, then stepped over the other boy and left the bathroom.
After making sure Katsuki was far enough away, Izuku got up from the ground and ran to pull the janitor's head out of the toilet, all the while clutching his aching shoulder.
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~
That janitor was Goto Imasuji, who a few months later would discover he possessed the quirk Muscle Augmentation and begin his new career as the villain Muscular, one of the most brutal and bloody assassins of our time, single-handedly destroying a gym with 26 Pro Heroes inside.
Oddly enough, he never sought revenge against the boy who drowned him in a public school bathroom.
...why am I writing notes in my journal about the flashback that happened in my mind?! Oh, well, never mind.