Well, BBS shows the Final Part of the Trinity*.
358/2 Days furthers Roxas, he is not that important in the grand scheme of things.
He has been explained in KH2.
*If you are wondering what this is, it is a name I made up for the three parts that make up everyone. The Body, The Heart, and The Soul.
Kimi wa ne tashika ni ano toki watashi no soba ni ita
Itsudatte itsudatte itsudatte
Sugu yoko de waratteita
Nakushitemo torimodosu kimi wo
I will never leave you