Kikyou is Sia twin sister that was meant to be born with her but she couldn't survive so Sia decided to save Kikyou by allowing Kikyou's soul to reside inside of her own. Kikyou emerges (takes control of Sia's body) when Sia is depressed or when she feels that Sia should be more aggressive to Rin. In Shuffle! Essence+, Kikyou gets an extended ending where she and Sia are able to be separated and thereby co-exist together. [from Wikipedia]