THEIR BROTHER IS THE NINJA HERO

Splinter was a Human that turned into a Rat and was mutated along with four Baby Turtles but as he's Taking care of them he hears Screams and fights off Criminals who attacked a couple and saw That the Man was Killed and The woman close to death and splinter recognized her from The Hamato clan she asked Splinter to take her son and Raise him and he Does along with the four Turtles and growing up Izuku had a fascination with Heroes and wanted to be one but he was always denied the chance to go to UA in Japan until one day he asks his father and he reluctantly lets him go as Izuku is Extremely Happy as He'll be The Ninja Hero