BTW, got a question, why won't Kyoka use her slave power on other girls? Won't they make a stronger slave than a normal guy than MC?
The rules of her ability stipulate that it can only be used on someone/something she can see as a “slave” and said “slave” has to be someone/something that can view her as a master. It has to be mutual otherwise it won’t work, which is why she used to force Shuuki to “submit” as is her catchphrase “time to submit!”
She forced them to view her as a superior and by proxy themselves as lesser. Since their ability for thought isn’t exactly there, it’s as simple as that. It’s the same with how the Humanoid Shuuki managed to establish a dominance hierarchy between themselves and their Shuuki.
The reason why Yuuki specifically is able to enter into a binding contract with Kyouka is because way back in the beginning he was someone who had been trained ever since he was young to view strong women as superiors. Thanks to his sister’s constant “training” he subconsciously linked the two, and since she was always bossing him around it established that trait within him which was eventually used by Kyouka to turn him into a “slave” she can’t do it on intelligent life without very specific parameters being met. And in this world no women would ever have the innate potential to become a “slave” like Yuuki did.
Maybe other men might, but at that point just stick with the one that’s proven capable and has grown very strong. It also helps that he gets along with everyone, and he now has a thunder god dwelling within him. Should that ever become known, his value within the MDC will skyrocket. However it would also put him in extreme danger, which is why the only people who know are Tenka and Kyouka two people he can trust more than anyone else. His master, and the one woman who does literally everything in the world for him, even giving him “rewards” without ever doing a “lending” she’s very specific when it comes to ensuring he knows she does it all because she wants to not because she has to like everyone else.
It’s what sets her apart from Shushu, she would rather take the easy way and force encounters through “lending” while Tenka makes a great effort to do it all without “lending” being a factor. It might be unfair to compare the two since their situations are entirely different as Shushu pointed out previously though