Complex sentence: Sora finds Kairi and Riku, whose appearance has been changed by the darkness to that of Xehanort's Heartless; he also reveals to Sora the nature of his connection to Roxas.
Split into simpler sentences: Sora finds Kairi and Riku, whose appearance has been changed by the darkness to that of Xehanort's Heartless. Rkiu explains to Sora the nature of his connection to Roxas.