: People who share the same goal and work/struggle/fight together in a group or organization. They often can be your close friends, too, but that's not necessary. A person whom you personally dislike, or whom you don't even know, can sometimes be your . In _One Piece_ it sounds dramatic because it's about people who share the same destiny, literally in the same boat. That won't happen often in real life. means "Join us," but not "Be my (personal) friend."
: A friend. Something based on personal feelings of intimacy or affection. You don't have to do something special with your . They can be someone you haven't met for a long time.