Are there any One Piece characters that are representative of Brazil?

Yes, according to the author himself, if Luffy lived in the real world, he'd be Brazilian!

How are the ethnicities of the characters in the anime One Piece determined?

According to what the author believes to be the closest to the character culturally speaking, he publishes it in the "Always Good to Know" section of the manga, where he usually shares official trivia about the story.

Discover All the Nationalities of One Piece's Characters