Implication on Rek'Sai's Bestial Nature: Understanding the Void
Given the current interaction of the void and its residents with Runeterra, we are able to glean a few truths about the nature of the void, its effect on biological sex, and the importance and roles of void creatures in the other realm.
Unlike Runeterra, who has similar patriarchal societies to Earth, the void governance may at first seem like some tribes of lore based on the Amazon. The women are stronger with males acting a play things for them, hunters which are useful for procreation and food. However, the actual effect of the void indicates this is not the case. As the environment of the void has definite effects on its inhabitants, the void (as a place of power) has had two distinct effects on persons from Runeterra with the main distinction being the biological sex of those offended and the roles being tied not only to the relationship of the void with these person but also on the roles of the sex of each.
First, Cho'Gath's lore indicates that Runeterra originally belonged to him and his brethren: "they were once driven from Runeterra by powerful magic lost to antiquity." This implies that over some time (as Cho'Gath is void born) that the reaction to sunlight is new to the voidborn and affects them aversely. At one point, the void ancestors lived on Runeterra on that sunlight and although they are not dying to the sun's rays (which would indicate a real problem with light making exposure unbearable or lethal), several of the creatures mention a burning reaction or the harshness (see, Cho'Gath, Vel'Koz).
This means that the void should share a common history with Runeterra regarding societal roles, particularly those regarding sex. Since Rek'Sai is considered monstrous and undeniably powerful and the first female beast champion, one might infer that the women of the voidborn are stupid, brutish, and through might are in control. However, this is probably not the case. Based on their common ancestry and the interaction of male and female humans with the void(Runeterran apparent humans, using term for simplicity), Void creatures likely have a similar preference for biological males than their counterpart but perhaps more specialized based on genetic markers.
The three Runeterrans to interact with the void are Malzahar, Kassadin, and Kassadin's daughter. The first is a prophet and even though the void could have many seers, Malzahar succumb to the call of void, a sweet voice akin to Cthu'lhu. The haunting nightmare did not destroy him but instead beckoned an end that enveloped whatever humanity might exist. Interestingly, his biological sex isn't telling as his greatness as a seer (see Malzahar's lore) permitted him to see such horrific visions, but rather the tell is his choice of sacrifice. True Kassadin's bloodline would make a suitable choice, Kassadin has already escaped the void's visions. Either Malzahar is scared (unlikely as he could team up with Cho'Gath who welcome the invasion of Valoran) or the void saw a female offspring more suitable. Given the power of the void, the latter is more likely.
So, the Void see specific values in biological sex, and I believe that the Void itself is able to speak to other through its interest. Imagine a hive-mind akin to the buggers of Ender's Game but using those with independent thought and free will. In a way, the Void is the same. The Void is able to spawn lesser creatures of no intelligence but only to consume those affected by its influence (See Malazhar's Voidling). Unlike LeBlanc, Shaco, or Orianna, Malzahar is unable to control his voidling, but instead it only attacks the most powerful influence of the Void affecting an enemy. it doesn't protect Malzahar by focusing those who hit him. It doesn't care. Instead, it attacks in a set order based on how close the void has infected the mind of a target. It follows a set path of the void's influence through Malzahar, but never Malzahar proper: Auto attacks, then malefic visions, and finally nether grasp. Malzahar is incapable of directly influencing the voidling.
So what does this mean?
First, the Void is more than a place. It is an entity that controls its inhabitants and makes them crave to return to it while simultaneously creating rifts on Valoran to increase its influence. Much like the Voidborn, the Void proper wants to consume Runeterra and every other dimension. The creatures of the Void are specialized for this purpose.
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The humans are pawns in a struggle to "appease" the Void. This is a show to pacify the citizens of all nations and open them up to the idea of a greater truth. This will inspire a cult following while making the discussion of the Void just commonplace enough to prevent any red flags from going off when real danger hits.
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Cho'Gath came as a scout. He is most capable of explaining his purpose when compared to other Void creatures. His hide and abilities allow him to deal with those who might immediately try to stop him, and he can set the way for articulating the sins of the ancestors of the current populace of Runeterra. He is the wrath of the creatures wronged by being sent to the Void while unwittingly allowing the follow-up of corruption by the Void. However, this is probably not so undesirable as the Void was his home for so long and he just wants Runeterra back. Whether or not it merges with the Void is not likely to be a massive concern.
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Kha'Zix is here to "devour the land's most promising creatures. Akin the Cell from DBZ, Kha'Zix is the Void's insurance against the most powerful beings in Runeterra. Starting small, he will consume the powerhouses of any future resistance and become strong by assimilation. He will amalgamate the greatest features of his foes until he has evolved and the Void's advance will become certainly inevitable.
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Kog'Maw is perhaps one of the most interestingly, cute, and terrifying creatures that Void has produced. Unlike Cho and Kha, Koggy is a devourer that is most directly responsible for preparing the coming of the Void. Kog'Maw eats color. He doesn't want your soul, vengenace, or even your physical being. In fact, he using bile and vomit to remove such physical properties and instead actively consumes the color of the grass, woods, and people. No one can stop his endless appetite a driving hunger for the most delicious colors Runeterra can offer. Interestingly, his rare taunt is paired with the far more common "Keep coming! Not full." This doesn't just indicate a voracious appetite but perhaps his taunt discusses not the coming of an actual father, but the father of all voidborn, the Void itself. "Terror coming . . . Daddy coming..." may still hint at a future champion (the ruler of the Void?) or it could indicate the impending doom. A goal once he has finished consuming.
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Rek'Sai is largely a mystery. However, I propose that her placement in the sands of Shurima may indicate she is protecting Icathia an the way to the Void's portals into Runeterra. More would need to be done to understand her purpose.
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Vel'Koz is the last one to discuss here, a hint at the one way to stop the void and an indicator as to its only fear: Zilean. Although Zileas is a favored troll of the forums, his counterpart is a troll for the void itself. Unlike every other inhabitant, he knows something that the Void doesn't. He knows the result. He has seen all time and is like God in this respect. Time doesn't really affect him and Vel'Koz's taunt indicates that an infinite number of samples would be required before any real calculations can be done for a simple battle never mind the final deciding moment. His task is to consume Zilean and learn, well, everything. He is driven by an instinct instilled by the Void: "Consume and learn." His compulsion to go to the tower (and the fact it is the first place he goes) means he has a mission he cannot remember. Nevertheless, it drives him forward to pursue an ultimate knowledge with no goal beyond that. Perhaps due to his intellect, he is prevented from knowing whereas Koggy pursues a goal. The Void, as this Vel'Koz's past indicates, does not care about Voidborn or Runeterra's denizens. Instead, it wishes to consume. Consume everything.
Conclusion
Rek'Sai is not mindless for any reason other than the void needs her to be. Her genetic disposition is not based on biological sex but instead indicates an underlying logic by the void that spawned her. Like every other one of her brethren, she is a tool being used by a dimension wishing to consume another. Interestingly, the magic of antiquity mentioned in Cho'Gath's lore might have been the catalyst for the Void's interest in conquest. Perhaps without the ancestors of Runeterra pushing out their fellows into the Void, the Void would not have known of Runeterra's existence. For more information, more research would need be done.