good post from Three Years Ago about giorno giovanna i found
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i've had difficulty articulating it, but this helped: if Josuke was meant to be a more human and ordinary protagonist, just a kid who wanted to live a normal fun life and keep his neighborhood safe, then giorno is meant to still be human but to be a person with extremely defined morals and ideals, and exceptional ambition to guide him.
Josuke is reactive: getting involved once the threat directly affects him. Giorno is proactive, actively scrutinizing every aspect of society and people, and getting pulled into conflict by his plan to address the root of the problem from the start.
And what defines this difference between them is their upbringing. Giorno learned early about the darkness in the world, and about the capacity of honorable gangsters to lift others out of it, regardless of what society says about criminals.
Josuke had a happy childhood and loving caregivers who instilled into him both a compassionate nature and a deep trust in society and the rule of law.
They both ended up fighting for something good, and they both could just as easily have fallen into a negative lifestyle: giorno becoming infected with the darkness that beat him down, and josuke's trust turning to complacency, his compassion to apathy.
Josuke has to do a lot of growing up over the course of the story, but Giorno's already had to grow up before his story begins. His arc is the uplifting of others, not his discovery of his own self.