Nami is a fantastic female lead. Almost deliberately, as the second member to join, she contrasts Zoro in the roles she brings to the crew.
Instead of strength, she offers brains. Instead of a kindred spirit, she's a counter balance to Luffy's antics. By that, she guides Luffy, both out on sea and in general situations. She reminds him not to be careless, not to make rash decisions, giving an out of the box support.
Of all the Strawhats, her loyalty for Luffy comes from the most heartfelt displays. Nami was introduced during Orange Town. That arc had heavy emphasis on treasure, and what treasure means to someone. Nami, the catburglar, as well as Buggy, were presented as valuing materialistic treasure. For Luffy, it was his Strawhat, and the powerful connection it holds between Luffy and Shanks. This theme carries towards Arlong Park, where we learn who were Nami's treasures this whole time: her home village. Luffy, considers Nami his "treasure" as well, for all he needed in order to face Arlong wasn't some sob story or deal, all he needed to know was that his nakama needed help.
Nami is arguably the most indebted Strawhat to Luffy for what he did for her. He shattered years of imprisonment, and gave her the opportunity to finally realize her dream.
Since so, Nami's expressed how important Luffy is to her, either through a general expression of loyalty with the others, wanting Usopp to develop a weapon for her so she can be of better service to Luffy, being the most emotional over training during the 2 years for her captain's sake, standing by Luffy's side alone in a Yonko's dangerous territory, and finally risking to meet death just because she cherishes the dream her captain strives for too much. Luffy has become a treasure to her.
Beyond that, she's just so much fun as a character. As the navigator, she's in charge of the day to day sailings, and everyone highly respects her skills and listen to what she has to say.
The crew is full of fools who wouldn't know what to do with themselves without her. And act out of line, you get a fist full of love which rivals Garp's.
These assertive traits are a possible adoption from her adopted mother, who was tough as nails marine woman, who took no nonsense from anyone. Once more, she has a soft spot for children, specifically abandoned ones, as seen when she refused to leave the children at Punk Hazard behind. Possibly, this is in relation to her being orphaned and understanding the difficulties of a child without a parent.
Nami has experienced countless perils, overcome many trials, and grown so much despite just being a relatively normal person. But this only makes her more relatable and endearing for facing the difficulties.
And did I mention she loves money? And loves to trick suckers? This sly cat.
