I just came up with a theory about the Void addressing why Kass is Darth Vader and Kai'Sa is sexy.

So, here's the theory: There are multiple "sections" to the Void, akin to rings of a hell. Kassadin glimpsed into a section that was pure insanity and destruction (like the Netherstorm from WoW.) This mere exposure to the energies of that place did spooky things to him.

However, Kai'Sa was deposited into a much less madness-inducing section of the Void. This would explain why she was able to survive without turning into an Ethereal.