A cording to what i saw and and am positive about 100% in the TGS 2009 trailer of BBS when aqua gives Terra and Ven a charm if you look closely in the center of it it has the emblem they were and it is the same when Aqua shows her charm in the video. I also found out that Aqua became an apprentice of Master Eraqus about a week to a month after Terra and ven some time after Aqua. If you noticed Master Eraqus' key-chain on his keyblade it is a platinum symbol me explanation for that is if you choose to be a master who takes on apprentices your symbol will change to platinum. Also masters may only have up to three apprentices the first one gets a gold symbol the second gets a silver symbol and the third gets a bronze symbol but these are not rankings they are they position you were apprenticed and won't change unless you take on apprentices. The design of the shape is created is chosen by the master who takes on apprentices, and that is my explanation feel free to comment this thread thanks bye.