Ok so I know Roxas is a nobody and stuff and so's Naminé but how does Roxas pretty much have a heart? Then for Naminé how does she have the ability to control memories when the only special thing about Kairi is that she's one of the princesses of heart? Or is Naminé able to do that just to make the story work? Also is Birth By Sleep a good game? I'm thinking about buying a psp just to play it but is there any point?
While Roxas counts as a special Nobody, Naminé is only called that because nobody (no pun intended here), not even Ansem the Wise, knows
exactly what she is.
That Roxas has a heart and can feel real emotions was pretty much already spilled out, however the "how" and why is (at least partly) explained and covered in Birth By Sleep, there you will also get to know why Roxas looks different from Sora.
Naminé does not have the power to control
all memories, she only has power over Sora's memories and over those of people connected to him.
It has also less to do with Kairi than with Sora. Since Kairi is a Princess of Heart and has therefore a Heart of pure light it is normally impossible for her to even give birth to a Nobody.
The creation of Naminé was only possible because Kairi's heart hid inside Sora's during KH 1. When Sora stabbed himself and became a Heartless his heart gave birth to Roxas and Kairi's gave birth to Naminé, but the body used in the process was Sora's.
Naminé was born from Kairi's heart but used Sora's body as an intermediary during the birth process, this linked her also with Sora and gave her the unique ability to mess with his memories.
If possible you should definitely get Birth by Sleep as there is actually a very big point to it. BBS is inofficially also known as "KH 0 (Zero)" and besides the main console games the most important entry in the series.
As DrHillboy already pointed out, it reveals a lot of essential stuff about the whole story, not to mention you are introduced there to three more very important characters and the main enemy of the whole series so far.
