When the Kitsune and Villin became one, Villin's control of his mental space and the magic power within it increased exponentially. It was also one of the things he never slacked on. Let's say the Kitsune had actually been an enemy, she was able to easily get through his defenses and destroy his mind, similar to what he did to the ogres.
Since he didn't want this, he was often busy increasing his defenses, and with increased control, he managed to do certain things that were previously impossible.
The rules of the mind weren't exactly the same as those outside of it. To some extent, one can form their own mind, it can be changed using your imagination and the control you have over magic.