He decided to spend some time by himself to draw out a larger-than-average mote of spiritual energy and apply some spiritual refining.
Using methods he developed before, he drew spiritual attributes that he didn't want and cut them away.
Ves tried to be as diligent as possible and focused mostly on cutting out the weird, unidentifiable alien attributes. Even if Ves didn't care about how William would turn out after being subject to contamination, it would be bad if the mech pilot started speaking an alien language or began to worship an alien god.
"There are still some limits to how far I can go. William still needs to be a functioning human being."
Therefore, Ves was fairly meticulous in his crude refining process. He kept cutting more and more undesirable parts out of the mote until it only retained a fraction of its essence.
Still, even this tiny fragment was enough to overpower William's spiritual potential.