I'm not saying don't complain about Kai'Sa, but keep in mind something...
She's not supposed to be as corrupted as Kassadin, Malzahar, or even Jax.
To survive among predators, Kai’Sa became one. Stranded for years with no hope of rescue, Kai’Sa endured through sheer will and a symbiotic second skin adapted from a living Void creature.
That seems to be more effective than what Kassadin did.
Cutting a burning swath through the darkest places of the world, Kassadin knows his days are numbered. Though once a notorious adventurer, he had chosen to live anonymously and raise a family among the nomad tribes of Shurima—until the day his daughter was claimed by the Void. Kassadin vowed to bring her back, combining a number of arcane artifacts and forbidden technologies for the struggle ahead. His precautions were not enough, however, and these devices now only serve to slow the insidious corruption that ravages his body... but he will gladly pay that price, if only he can save her...
As for the suit itself, they could've went with a Venom or Metroid style of living suit, but because she's this high speed marksman, Riot went with a sleeker design. It's like Gundams vs. Mobile Suits, at least the ones the main protagonists tend to use.
At first, I was triggered by the cleavage, but apparently she can cover that up.
https://cdn.discordapp.com/attachments/318134046276321280/415557768985051136/kaisa.PNG
Overall, I like her, and think people should asses things more carefully before getting upset.
EDIT: To put it in another way, Kassadin is barely keeping it together. Malzahar is mentally warped. Jax is physically corrupted. Kai'Sa adapted to the Void. She probably mastered it.