User blog comment:JapaneseOPfan/Lesson 12 or something (Pronunciation)/@comment-24568337-20150814192507/@comment-3345720-20150816161531

In the case of One Piece, pretty much all names are in katakana, even the Japanese ones like Nami.

Also, a lot of words in Japanese are derived from foreign languages (piano, lemon, bread [pan], just to name a few), and you also often use katakana for sound effects and in random places to convey "foreign-ness" (like when foreigners, aliens, and robots talk, or how the word "sun" is written in katakana when the fishmen say it in One Piece, since the sun is something "foreign" to them)