#another theory is the senpai kouhai thing coz sanji is her junior in terms of joining order

[kuroananosanji]

Idk how to explain this but even though “-kun” suffix suggests distance it’s also used when women wanna seem feminine/cute? Like you’d call your male classmates “-kun” if you wanna maintain a casual yet polite image, while you’ll pretty much only address your brothers by their raw name because conventions and allat don’t matter. So it seems like Nami doesn’t care about her lady image in front of other male SHs at all but in front of sanji she likes to be cute to a certain degree