I'm pretty sure that Sora will be able to save them all. Roxas, Namine, Xion, Axel, Aqua, Terra, and Ven all have different kinds of torment. Roxas and Xion went back to Sora even though they wanted to be their own people. Aqua is trapped in the realm of darkness. Terra got body-snatched by MX. Ven's body is somewhere in CO without a heart. Axel is dead and had to let go of his best friend, Roxas. Namine never had a chance to do what she wanted to do and was controlled her whole life. Sora has to save them because he connections with most of these people in some way and because Sora has this ability to connect his heart with anyone. Riku has a connection with Xion, Roxas, Namine, and Terra. Kairi has a connection with Aqua, Namine, Roxas, and Xion.