Born Haruno Shiobana, Giorno is the son of Dio (fathered using Jonathan Joestar's body, which logically that would make him part of the Joestar family) and a Japanese woman. As a young child his mother remarried, to an Italian man, and they moved to Italy; hence Haruno Shiobana became Giorno Giovanna.
Giorno's great ambition in life is to become a mafia boss (a "Gang Star"), and as such he eventually joins the mob squad Passione. Though he is its youngest and newest member, he quickly proves his mettle and eventually becomes the de facto leader of the group.
Unlike typical mafia bosses, he wanted to give divested riches to the poor, giving him an agenda similar to Robin Hood. He eventually influences all of Passione, all but their boss, who later became their worst enemy.