Des is one of Mary Harris' assistants. He is the cameraman and general nurse for Mary's patients, ensuring that their consent is captured on camera when they are ready to die.
Des was previously addicted to and abused an unknown drug. He is now sober, though he mentions in conversation with Mary that he does miss his drug addicted lifestyle because being sober bores him. He uses his past lifestyle in order to procure drugs from Grady, who believes that Des is still abusing drugs and uses the phenobarbital for himself.
Des believes that he and Mary should instead begin injecting the phenobarbital rather than administer it as a drink. He has mentioned Troy Dixon's botched death as a reason multiple times. Mary always refuses, believing that an injection removes the patient's choice in the matter.
Des doesn't like being responsible for anyone other than himself; in The River Styx, he has already "lost" Mary's cat by leaving a window open after only one day.
Des appears in all episodes of the series.