An angel, he is based upon the angel of silence in the Kabbalah. He first appeared in The Sandman: Season of Mists, after Lucifer closed Hell and gave the key to Morpheus.
Eventually, after requests and demands from a multitude of entities for the property, a message is delivered to Dream via Remiel; "Hell cannot be entrusted to other than those who serve the Name directly...myself and Duma are to take over Hell." Stunned by the edict, Remiel immediately rails against the decision, forcing Duma to be the one to actually accept the key from Dream.
Ashamed, Remiel told Dream "I cannot allow my fellow to drink form a cup I have refused. I will go with Duma." The pair then traveled to Hell and reopened the gates, taking up their positions as caretakers.
Other Appearances
Duma later appears in The Kindly Ones where Lucifer mentions that he respects Duma. Remiel sourly told him that Duma, the angel of silence, can talk to Lucifer if he wants to.
He also appeared in The Wake and at Dream's funeral, he gets up to "speak", but maintained his silence, simply shedding a single tear, which effectively communicates his feelings to all those present.
He and Remiel reappear in the spin off series Lucifer, wherein, Duma actually does speak when he defies Remiel, and says that Christopher Rudd was entitled to be the caretaker of Hell. Duma, who still had possession of the key to Hell, takes it from around his neck and hands it over to Christopher. Remiel was shocked to hear him speak, and said states that he has forsaken God by breaking his silence. Duma retorts that he has chosen a new type of servitude and remains by Rudd's side as an adviser.