Nami is Luffy D. Monkey's navigator, formally a renowned female thief that would stole only from pirates. She's quite greedy and hot tempered, but is also very intelligent and kind, occasionally. She is anything but interested in joining Luffy's crew at first. But after an incident with Buggy the Clown, joins up with him on a temporary basis.
Wikipedia contains spoilers
Nami is an orphan that was found by a female Marine on a battlefield as an infant. The marine adopted her along with another child and the three became as close as a real family. However, this ended when the infamous Merman Pirate Arlong took over their village.
Because Nami's mother could not pay for her entire families' monthly fee for living, Nami's mother resisted Arlong and was killed. Then Nami was kidnapped and forced to become a Map Maker for Arlong. However Arlong made struck a deal with Nami, if she brought him one hundred million Belli, he would free her village.
She spent several years making maps and stealing the treasure from Pirates in order to buy back her village. Luffy was just another victim. After a while she stole the treasure Luffy had accumulated and returned to her village. With this treasure she almost had enough money to buy back her village, however Arlong arranged for a corrupt marine to confiscate her treasure and she was left with nothing.
Later, after Luffy dealt with Arlong and Nami's village was freed, Nami rejoined Luffy's crew as a permanent member.