kairi and sora were linked not only because of kairi's love for sora but because of there trust. Remember when she wanted to leave the island with only sora (no riku).... hence the trust. he was there for her when her haert was weakest and that is probobly (along with the fact that he was pure of darkness) why she took refuge inside of him. Perhapse if riku had stayed and been at that spot, at that time she may have entered his heart.