Zach was one of Eli's many victims who was hunted down because of his status as an EO. His ability involved being able to see beings, possibly ghosts, that others could not see.
He was involved in a mine tunnel collapse a year before meeting Eli and Serena where he suffocated to death. He woke up with an unspecified ability to be able to see things others could not, which caused him to appear mentally unstable to others and was prescribed antipsychotics. Because of this ability, he went from being married with three children to divorced and unemployed. While initially his behavior was due to his ability rather than mental instability, his ability ended up taking it's toll on his mental health too.
Serena and Eli go to visit him in order to assess whether or not he is an EO. Serena notes that his home and front yard are unkempt and concludes that he lives alone.
He is coaxed out of his home by Serena and tells them about his death and how he didn't want to die alone and begs to make "them" stop and has not been able to make "them" go away ever since they started appearing to him.
As Serena is about to ask him what he means, Eli shoots him in the head, since they had confirmed that he was an EO, claiming that it was a mercy.