A special bond
In One Piece, before the integration in the strawhat crew, a special bond is created between each Nakama and Luffy. This bond lead to one thing, Luffy stating in one way or another that the character is or will be a Nakama. This bond consist in two very simple thing: A strong connection from Luffy toward the Nakama
And
A strong connection from the Nakama to Luffy The character that will integrate the crew must therefore depict an extrem moral attraction toward the character of Luffy, enough to follow him and same for Luffy.