Guillermo was a cardinal of the Radiant Church. He personally paid Linley a visit after he found out about Linley's talent, having been a dual-element magus.
He initially helps Linley by providing him with guards and books on high level magic.
After he learns of Linley's hatred for the Radiant Church, all he can do is sigh and obey the orders of the Church as he hears of their plan to terminate Linley.
He is later seen leading the Radiant Church's army in a campaign against the newly formed Baruch Kingdom, joint by the forces of the Cult of Shadows in the Anarchic Lands.
After he almost secures the victory over the army of the Baruch Kingdom, they are defeated by the Ratmaggedon wave which completely wiped out both the armies of the Radiant Church and the Cult of Shadows.