William's so-called "battle of brains" had actually relied on a little trick.
This method of placing brain cells into a container was indeed related to the brain, but it also involved another important ability—creating clones.
William had stayed in the City of Evil for a long time and had entered and exited Mobius many times. He knew very well that although Ruben was also skilled at creating clones, he mostly relied on similar individuals selected by the Mental Hospital to occupy bodies. He rarely created clones through the division of his own cells.
As for William himself, as the sole heir of the evil cancer, his ability to create clones had been fully maxed out through the perfect division of Cancer Cell.
Not only that, but to obtain a more perfect "combined clone," William had also made use of the flock's reproductive abilities.
Just as the "offspring" he and Atpei had once cultivated had been able to invade the Bloodfolk Manor, William had used the special Cancer Cell he developed together with Little Grape's brain cells for joint cultivation. By incorporating the concept of the Birth Gate, he had obtained offspring cells.
He then used external modifications to disguise them as an ordinary mass of brain cells.
William had prepared all of this in advance. He seemed to have been certain from the beginning that Ruben would accept the performance, at which point he would bring out this mass of offspring cells to begin the first game.
The so-called "third party" providing the "vessel" had also been specially manufactured in Garp's laboratory. It could perfectly withstand the Evil Sun's properties, while every part of its body could dissipate heat with high efficiency.
The game character ultimately created could completely be regarded as the perfect fusion of William and Little Grape.