William Ennis was an employee at the Sierra Madre.
A locker called Ennis's locker can be found and Ennis's key which according to a computer log he "left his key on one of the consoles again ". In the same log he is referred to as "old man Ennis". His locker contains a note titled toxic samples which suggests that Ennis was one of the workers in the hazard suits to protect the Sierra Madre against the Cloud.
Appearances
William Ennis is only mentioned in the Fallout: New Vegas add-on Dead Money.