After defeating Lady Alvida, Luffy and Koby escaped on a lifeboat. As Luffy's been telling Koby that his goal is to search for the "One Piece" treasure in order to become the Pirate King. And he'll be needing a big crew if he wants his dream to become a reality. Koby also has a dream, and that's becoming a marine so he could capture ruthless pirates like Lady Alvida. Luffy understands Koby's dream pretty well, and will make sure that he'll become a marine.