Kai'Sa's in-client lore is out
Claimed by the Void when she was only a child, Kai'Sa managed to survive through sheer tenacity and strength of will. Her experiences have made her a deadly hunter and, to some, the harbinger of a future they would rather not live to see. Having entered into an uneasy symbiosis with a living Void carapace, the time will soon come when she must decide whether to forgive those mortals who would call her a monster, and defeat the coming darkness together... or simply to forget, as the Void consumes the world that left her behind."
Given her VO, it's overwhelmingly obvious which of those options she's going with.