(In my heart, Zero has always been the sixteen-year-old boy, Morohoshi. This story takes place in a parallel world, so Zero's character may not be that flamboyant.)
Twelve years ago, eight-year-old Yuka Asano was rescued by a boy from a disaster caused by a monster, but lost her mother. She witnessed the boy transform into a red and blue giant and defeated the monster, and she remembered his face ever since. Twelve years later, Yusha was once again attacked by a monster, and once again met the giant who saved her. But this time, he was defeated by the monster...
Ultraman is the patron saint? Yes and no. They have power, but they cannot save everyone.
But that doesn't stop them from becoming heroes.
Concept: There are no perfect heroes, but heroes are always great.