What if the Void isn't as bad of a place as we thought(its still pretty shitty tho)?
Just think about it. I don't think void creatures treat each other like they would treat human beings. If that were true, it would mean that beings like Cho, Vel and eventually Kha'Zix would be some of the most powerful creatures on that realm. There has be some type of hierarchy and naturally there would be some kind of "lesser void-being" resistance that are not happy with how things are.
Everyone's complaining how Kai'sa isn't more monstrous but eveeeeryoone loves Parasyte lol. But i'm sure void creatures want to live and survive just as much as humans, so I can believe that a void creature was in desperate need of a host found Kai'sa close to death and decided that working together would be the best way to survive.