Um... this may be an old forum but, I have my thoughts on Roxas' symbol. If you think that the Organization symbol is actually an X and a heart... that may symbolize they have no hearts. Since Roxas doesn't have the heart piece in his symbol, he may have somewhat of a heart? ...And the X may mean his existence is about to end... That's all.
That heart symbol of Birth By Sleep is like an ID for Keyblade Master in training. That's what I think.