I just read it from Nami's end, and, to me, Nami acts like Luffy's wife. Nags him, worries about him, believes in him, and, as far as tag team fights, Nami is in her own category with Luffy.
Luffy and Nami vs Enel, and Luffy and Nami vs Cracker, pretty much proved to me that Nami is Luffy's...