Kai'Sa is not sexualized!!
She is just HOT AS HELL, and just trying to cool off after a hard days work saving your ungrateful lives. She is a human after all, and one of the greatest evolutionary achievements of humans is the ability to sweat! Considering she is constantly fighting for her life in the void, and running marathons while doing it without a cool refreshing bit of iced tea, it should be no surprise that she is working up a sweat. Hell most of us would start sweating just walking to our car, I personally have started sweating while locking my front door to get to the car, partially because TEXAS, but mostly because I am out of shape.
AS WE ALL KNOW, the brain and heart are some of the biggest sources of heat in the human body, so she HAS to expose her head, and she HAS to expose a large portion of her chest in order to cool her self down after a hard days work of fighting the voidpacolyps!
But it is not just her own body heat that she is trying to regulate. OH NO IT IS NOT! She also has to regulate the body heat being produced by the LIVING REACTOR that is her bio suit. After all, I highly doubt that void powered creatures who constantly produce energy, at rates that would make a power plant weep from jealousy, cares little about needing to regulate body temperatures the same way that humans need to. I am amazed she hasn't died from dehydration yet considering she has TWO VOID JET ENGINES just sitting on her shoulders, while a void reactor is PROPELLING HER TO CONSTANT LIFE AND DEATH SITUATIONS, and she can ONLY expose her head and a large portion of her chest. This woman needs some ice cubes to rub herself with so she can finally have some relief.
Plus with all those void creatures being powered by pure energy, I am more than certain, that the void is hotter than any of us could hope to experience on any given night with that special someone.
TLDR; You go for a long run on a hot day, and than don't take your shirt off when you are done! Also NO A/C! See how quick you try and turn your shirt into a V neck.
P.S. To all those haters out there. YOU make her sexualized, because YOU can't separate sex from the equation. Please become more mature.