Major player on WCI, key emotional scenes and also decision maker.
Decision maker on Zou.
This is what you call prominence, and panel presence. Anytime you see these internal straw hat moments, Nami is highlighted in majority of them, one way or another her input is considered. Been this way since the very beginning of the series. Robin is borderline a background character in comparison.
You can easily argue Nami is the "2nd lead" of this series by her overall panel time, and panel presence. Each of Zoro, Sanji, Nami have an argument of their own.