Ok, to get Nami's 2nd costume, first you must have the map area that has the Large Whirlpool (Called the Jaya sea area) in Luffy's story mode. Go there and choose Vermillion town; you fight Buggy there. The bonus says that you have to defeat Buggy while being Nami. Make sure she's lv 20, or better yet higher than lv 20 for her HP/Attack/Defense are very poor. After you beat Buggy, it'll announce that you've unlocked her costume. Now you can wear it in battle mode. :) Hope this supports people who were looking for this. See ya later!