In an SBS question, a fan asked if the Straw Hats were real, what nationality would they be of. Based on their appearances, Oda gave his answer.
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D: “Oda-sensei, I have a question. If One Piece was set in the real world, which countries would the nine Straw Hats come from?”
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Oda: “Well. I’ll just put what fits with the character’s image.”
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Luffy: Brazil
Zoro: Japan
Nami: Sweden
Usopp: Africa (South Africa?)
Sanji: France
Chopper: Canada
Robin: Russia
Franky: USA
Brook: Austria
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Curiously enough, Usopp doesn’t get his own nationality, but instead we have to guess where in the continent of Africa he might be from—maybe he’s South African?