First introduced in Industrial Magic, he is the son of Benicio Cortez, and middle brother of Hector and Carlos Cortez, and half-brother to Lucas Cortez. He is described as being about 70 lbs overweight, and often shows resentment towards Lucas because of the fact that Benicio clearly favours Lucas out of his sons. After harassing Paige Winterbourne, he fell victim to one of her suffocation spells (although she didn't kill him with it) to which Carlos remarked that he'd never seen witch magic like that before. He died in Personal Demon.
In Dime Store Magic, Lucas describes him as intelligent, but unwilling or unable to take initiative. In Personal Demon, Lucas states that William is his favorite brother, though this only means that he is the only one who hasn't tried to kill him. In this novel, Jasper Haig, when speaking to Hope Adams, states that William and his brother Hector are intelligent enough to one day take over the company, but are only "good enough" and not smart enough to make the Cortez Cabal better, which is why Benicio wants Lucas to one day take over the cabal. This contrasts with the description Lucas gives him in The Case of El Chupacabra, where he states that while Hector inherited their father's intelligence and ruthlessness, and Carlos inherited only the ruthlessness, William unfortunately inherited neither, though he could have been referring to William not taking initiative. This is supported by Jasper's description of Benicio, where he states that the Cortez Cabal has not always been the most powerful cabal, but Benicio made it so after he took over, and intends someone capable of continuing this to run the cabal after he is no longer able to do so.