Thanks to unique character designs and mannerisms, so many One Piece characters are memorable and iconic in their own way. The Straw Hat navigator Nami is no exception to this.
In fact, her iconic style has made Nami more recognizable than fans think. Her trademark lack-of-shirt outfit in the New World is particularly what non-fans of the series best know her for.
While Nami does have some outfits which incorporate a shirt or something covering the equivalent space, she’s mostly seen in her bikini top. While Oda has given One Piece fans no official rationale for this, the community has come up with a solid theory as an explanation.
A popular fan theory provides a great explanation for Nami's outfit choices throughout One Piece
One Piece fans have begun theorizing that the answer to “why does Nami not wear a shirt” is related to her unique weather sensing abilities.
Since her introduction, Nami has been shown to have an incredible, naturally gifted feel for the weather. Fans have consistently seen Nami predict the Grand Line’s weather since the Straw Hats entered it. Throughout One Piece, the Grand Line weather is often characterized as wild and unpredictable, so this inherent ability of Nami’s is truly uncanny.
The theory also has good evidence behind it as well. For example, in One Piece’s Water 7 arc, Nami changes into an outfit consisting of a skirt, bikini top, and jean jacket during the Aqua Laguna. While this outfit isn’t necessarily going to keep one dry or warm during a massive storm, it will help Nami feel and predict weather patterns better and more accurately.
While fan-service certainly plays a role in Nami’s character design, writing the choices around this theory makes Nami one of the most talented people in One Piece. It also makes her even more of a unique, irreplaceable member of the crew than she already is.
We already know Nami’s navigation skill and cognizance for weather is unique; even the weather experts of Weatheria were dumbstruck by Nami’s abilities. The Straw Hat crew is already top tier in One Piece, but to have someone who can predict weather, that is hailed universally as unpredictable, is a great ace indeed.
While fan-service certainly plays a role, her unique weather sensory skills not seen elsewhere in One Piece must influence her outfit choices. By exposing more bare skin, Nami would be able to sense the air around her and its subtle changes better.
Evidence for this claim exists both in both pre and post-timeskip One Piece. Nami’s Aqua Laguna outfit and her wearing a bikini whenever sailing the New World, respectively, are the key supports to this theory.
Considering the wit of Oda, it can’t be the only factor. But for now, there’s no doubt that Nami’s unique skill, found nowhere else in One Piece, is the key difference-maker here.