As we know by the full ending, Ventus's heart or life essence meets little Sora and little Sora being caring makes a little agreement for Ventus to stay inside him. This brings me to a theory.
Terra chose Riku to be his apprentice and successor. I think that Ven being inside Sora for a long time and seeing how of a pre hearted person he is chose Sora to be his apprentice and successor. The voice Sora hears in his mind is Ventus and the beginning of KH1 was a training session made by Ven.
Terra chose Riku to be his apprentice and successor. I think that Ven being inside Sora for a long time and seeing how of a pre hearted person he is chose Sora to be his apprentice and successor. The voice Sora hears in his mind is Ventus and the beginning of KH1 was a training session made by Ven.