Nami was the second person Luffy asked to join his crew, but she technically became the fourth member. At the time they met her, she was a thief who stole from pirates, but decided to accompany Luffy and Zoro for her own benefit. Soon after arriving to the Baratie, Nami stole the crew's ship and returned to her home by herself. Despite this, Luffy never stopped trusting her, already thinking of Nami as their navigator.

The truth was she worked for the merman captain pirate Arlong, who murdered her foster mother when she was a child, and decided she could be of use with her amazing navigational skills. Arlong owned Nami's home island and its inhabitants by all means, so she made a contract with him: she would obtain enough money to buy the island from him and free her people. Of course, Arlong ended up breaking his promise, and Nami, finally hopeless, had no choice but to ask Luffy and the others for help. They gladly took care of Arlong and so Nami became the crew's navigator.