They cant brainwash him with the lending. He lost his mind the first time, but he was just attacking everything in sight, if she were to give him her power she would be at a disadventage.prediction for the next chapter would usually go something like: Chief Ren is able to take over Yuuki with her sheer power and make him fight Kyouka.
Ms. "I'll make you my slave using my slaving ability" vs Ms. "I'll make you my slave regardless"
It likely would, but I doubt Yuuki would do it. He already told Ren that he doesn't intend to use her "dog rewards" against her and that he'd help Kyouka win without relying on it.If Yuuki reveals all her embarrassing "dog Rewards". that would throw her off her game.