According to SE "http://support.jp.square-enix.com/faqarticle.php?kid=67225&id=5382&la=1"--> you need REGISTRATION CODES to play multiple characters on the same account.

I have a main SE ID account but under mog station, it also has an option to add new service account. Does it mean that I have to buy a new game out of the box to get the code?....also if I do buy the game and get new code, will the new code be able to apply to the same service account that I have or do I have to add a new service account on mog station........ What I really want is to have me and my cousin be able to play 2 characters I have on my SE account on the same server.....but I am hesitating to buy another game copy to get the code and then each month I have to pay double to subcription for 2 game service.....which cost like 26 dollars a month.