JoJo's Bizarre Adventure has rather odd storylines that don't always go into some of the finer details of the series. This is where fans have stepped up to the plate, creating compelling theories about the series. Some of the best theories are so good it's hard to imagine that they aren't canon.
Both the manga and anime ofJoJo's Bizarre Adventure focus heavily on combat and the adventures its main characters go on, leaving little room for character development. This leads many show fans to create theories to connect the stories, develop the characters, and further the storyline. The top theories cover small details of the story, but after hearing the reasoning, it's not that far of a stretch for these ideas to be reality.
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Star Platinum is Jotaro Kujo's stand, with powers the same as Dio's stand, The World. Initially, this theory sounds ridiculous; at the beginning of part 3, it's explained that stands are the extension of a person's fighting spirit. So, this would mean that Star Platinum is an extension of Jotaro, but the Joestars are different. They got their stands after Dio emerged with Jonathan's body, activating their stand, different from other stand users who are born with it or gain it from an arrow. The evidence that fans use in addition to this technicality comes from what Jonathan once said to Dio and Star Platinum's appearance.
During part one, when battling Dio, Jonathan proclaims that his ghost would haunt Dio if he died. Knowing Jonathan Joestar, this wouldn't have been an empty threat; Jonathan's spirit would have kept tabs on Dio. Meaning that after his return, Jonathan would come back to stop Dio; what better than as a stand.
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