I just don't get it.
People have had a lot of misconceptions about what the Void is and what kind of creatures inhabit it. And we've had examples like Cho and the like who laid the foundations on what kind of creatures we should expect to see as Voidborn. But we also had (to now) 2 humans and one Vastaya who came in contact with the Void and they haven't been mutated to a point of becoming the aberrations we've grown accustomed to, only driven by their instinct to consume (and adapt).
Considering that Kai'Sa is Kassadin's daughter (the same one Malzahar used as a sacrifice to the Void - if i were to believe her interactions video), she's human and her suit (aka symbiote) allowed her to survive the harsh environment of the Void and survive for years by adapting. So her symbiote is her Void mutation because she can't get rid of it that easily.
It's like her second skin. It lives and breathes on her body and in turn, she can expand her capabilities as a predator from the Void and infuse it with the Voidspawn abilities that makes her quite effective at dealing with the monstrosities themselves.
She's back into the real world that she came from and her task is to protect everyone from the Void and its intrusion(s). Void is like a power trying to consume all living things and turn them into the reflection of itself. Kassadin shares the same ideals because he allowed his visions of the Void to change him BUT he overcame it and now uses his newfound power to fight the Void. Malzahar is the exact opposite. He welcomes the Void and is willing to do anything to make his vision a reality.
Hell, even Aurelion Sol (in his lore) himself also thinks that Voidspawn have no business threading in his dimension and he is more then willing at showing them just a faction of his power to wipe them out like the dimensional perversions they are.
Vel'Koz is also quite different then your regular Voidborn as well.
He's actually highly intelligent and his main concern is disintegration of matter to gain the knowledge of what it's made out of, in what configuration it is and how does it perform its functions (what is its function at all?). And he'll even turn on his own if he ever feels like his own pursuit of knowledge is threatened by his instinct-driven companions that are only worried to sate their hunger.
The complaints i'm hearing are that she's too "pretty" to be from the Void, that her VO is quite meek and that the expectation was that she should be angry or hellbent on revenge for some reason. Like, you don't even know her personality or her story and you've already made up your mind on how a Void champion's script should play out in your head?
Like i said, many preconceptions and misconceptions have been had and people just don't want to admit that to themselves. And furthermore, a Void champion is what i was expecting after Vel'Koz and I AM NOT disappointed in Kai'Sa at all. The void did not give birth to her. It gave birth to her symbiotic suit that probably at some point, was a Voidspawn much like Kha'Zix. The Void MADE HER who she is now and he has retained her sanity and mind. Now she's come back to protect the people from the world she's from, against the ever-approaching Void intrusion.