William is a minor character in Season 2 of Fear, who appears in a flashback.
Not a lot is known about William's life. However, at some point, he met Ofelia Salazar, they started dating, and they fell in love, eventually prompting him to propose to her. It is unknown how long they were engaged for before it was broken.
In the episode Pillar of Salt, William and Ofelia are seen enjoying a meal at a seaside restaurant. After the waitress clears their table, she brings out two glasses of water, one of which contains a ring. William proposes to Ofelia. He also asks her to move to Santa Fe with him as he starts a new job. While Ofelia is hesitant, she is excited and agrees to talk to her mother about marrying William.
It is unknown what happened to William after the apocalypse started. It can be assumed he was in Santa Fe when the apocalypse started. It is unknown whether he made it out of the city alive, died, or became infected. It is conceivable he remained in the city, and may have died when/if the city was bombed in accordance with Operation Cobalt.